According to Protestants, why was Mary troubled at the greeting of Gabriel? We read the following in Luke's Gospel (1:26-38 DRB):


  And in the sixth month, the angel Gabriel was sent from God into a city of Galilee, called Nazareth, to a virgin espoused to a man whose name was Joseph, of the house of David; and the virgin's name was Mary. And the angel being come in, said unto her: Hail, [kecharitomene], the Lord is with thee: blessed art thou among women. Who having heard, was troubled at his saying, and thought with herself what manner of salutation this should be. And the angel said to her: Fear not, Mary, for thou hast found grace with God. Behold thou shalt conceive in thy womb, and shalt bring forth a son; and thou shalt call his name Jesus. He shall be great, and shall be called the Son of the most High; and the Lord God shall give unto him the throne of David his father; and he shall reign in the house of Jacob for ever. And of his kingdom there shall be no end. And Mary said to the angel: How shall this be done, because I know not man? And the angel answering, said to her: The Holy Ghost shall come upon thee, and the power of the most High shall overshadow thee. And therefore also the Holy which shall be born of thee shall be called the Son of God. And behold thy cousin Elizabeth, she also hath conceived a son in her old age; and this is the sixth month with her that is called barren: Because no word shall be impossible with God. And Mary said: Behold the handmaid of the Lord; be it done to me according to thy word. And the angel departed from her.




Question

My question is two-fold:


Was Mary troubled at


a) the fact of Gabriel's speaking with her, or
b) what he said to her (i.e. the "manner of" the greeting)?

If (b), what does this imply for Protestants about a) Mary, and b) the uniqueness of the (manner of) greeting as relates to other angelic visitations in Scripture (if any).




Thank you in advance.
Part II - Mary, and the uniqueness of greeting.
1) Mary, Herself

Mary's place in scripture is very clearly that of a woman greatly blessed by being chosen to perform a quite unique service, but not to be promoted into a unique office. Gabriel's greeting underlines this, 'Highly favoured, among women'. She remains among women, as one of them, and is highly favoured.
Jesus never addresses Mary as mother. He calls her, 'Woman,' when she speaks to him in Cana at the wedding and he calls her, 'Woman,' from the cross when making provision for her with John. When Mary slips in her words and speaks of, 'Thy father and I,' she receives a mild rebuke from the twelve year old boy, 'Wist ye not that I must be about my Father's business '

2) The Uniqueness of Circumstance

(Seven Appearances)
The LORD appeared to Abraham in the plains of Mamre, Genesis 18:1, and he looked and Lo, three men. 'My Lord,' he said. And the unnamed men rose up from thence. But the LORD continued to speak to Abraham. A messenger, malak, spoke to Moses out of a burning bush. The LORD saw and God called, Genesis 3:4. An unnamed angel spoke to the unnamed wife of Manoah, Judges 13, and later ascended up in the flame of the altar.
A man strove with Jacob till the breaking of the day and would not tell him his name, but he called Jacob 'Isra-el' for Jacob had power with God and had prevailed. Daniel saw the appearance of a man and he heard a man's voice say, 'Gabriel, make this one to understand. Later, Gabriel appears and says, 'O Daniel . . . .' One appeared to Daniel and told him, Daniel 10:13, that he was helped by Michael, the archangel.
And the greatest king upon earth, at the time, Nebuchadnezzar, saw four men in a furnace of flame, Daniel 3:25, and said, 'The form of the fourth is like unto the Son of God.' These seven occurrences are singular visitations of huge importance in the unfolding  revelation of the purposes of God upon earth. They rise like seven pinnacles over the rest of the comparatively trivial events recorded by humanity of its own history.
And then, after Daniel and Malachi, comes silence. Not a word, for four hundred years. But there are a few faithful, such as Simeon and Anna, Zacharias and Elizabeth - and Joseph and Mary.
Then comes not just an angel but an archangel. And not unnamed - this is Gabriel who announces to Zacharias, 'I am Gabriel, who stand in the presence of God - -  and behold thou shalt be dumb !' He has the power to strike a man dumb if his herald is not believed.
Michael is also called, apostolically by Jude, an archangel, Jude 9. John's visions indicate that Michael fights with the Lord's hosts, Revelation 12:7. But Gabriel stands in God's very presence.
There are three that appear significantly - two with personal names, Gabriel and Michael, and one who is only ever represented in ten descriptive terms, as Serpent, Satan, Diabolos, Antidikos, Poneros, Abbadon, Kategoros, Beelzebul, Apollyon, Lucifer, Animos and Drakon. This one is defeated by Michael, once it is righteously proper to do so.

3) The Uniqueness of Greeting

Only to Mary is it said that 'The Holy Spirit shall come upon thee and the Power of the Highest shall overshadow thee,' and only - ever - of that within Mary was it, or could it be, said that, 'Therefore, also, shall that generated holy be called Son of God.' [Literal Translation of TR.]
These words, and the circumstances of them, stand out even against the background of the seven previous singularities recorded in scripture. This event is breathtakingly momentous.
My wording - “generated holy ” - reflects the exactitude of the wording used, gennwmenon agion, of this momentous event . The English translation is unsatisfactory, even in the AV which I, otherwise, greatly respect. The wording comparison in Matthew and Luke is just stunning, utterly stunning, and deserves to be further highlighted.
[Edited after Post in order to keep closer to the text and to the answer required.]