Why is 当然 the correct choice in 我学了10年汉语了，汉语水平 [一定 / 当然] 很高? I recently had to select the correct answer for a homework assignment:


  我学了10年汉语了，汉语水平 。。。 很高


I chose 一定，but the correct answer turned out to be 当然

Was my answer incorrect, why?

How do these two answers differ in meaning. I'm thinking perhaps that 当然 has a stronger connotation of certainty, as in yes my Chinese level is very high, where as 一定 implies that it should be high, but isn't necessarily... Is this correct?
一定 either introduces an INFERENCE, which is very likely to be true (as in the context of the given sentence), or shows a strong intention (in other contexts). On the other hand, 当然, equivalent to the English 'of course', indicates an inevitable consequence that the speaker KNOWS to be true. 

In this sentence, since the speaker is talking about himself, he does not need to make an inference about him being good at Chinese; he knows he is good at Chinese. Therefore 当然 would make more sense here.

An easy way to differentiate the two is to translate the sentence into English with 一定 and 当然 replaced by 'must be' and 'of course', respectively, and you will be able to immediately see which one is better.