What is the meaning of 为零 here？ I had the question: 1994 到 2004 年间，不能自理的老年人怎么样？

2 answer choices were:


  B. 比例下降
  D. 比例为零


I see that 零 can mean 'wither and fall'. In English we can say e.g. 'The numbers have withered.' to mean a fairly drastic reduction in numbers of something.

My question is then: are B and D the same answer using different words, or does 为零 mean 'nearly zero, as zero'?
为= being / is

零= zero

Answer D: 比例为零 (proportion is zero) means the proportion of elderly people who can't take care of themselves in the population is zero. (no more elderly people who can't take care of themselves) . That makes no sense. 

The writer of the question might had phrased it wrong, or the answer is just obviously incorrect.

Answer D should had been 比例增减为零(proportion increase or decrease is zero) which mean 'no change'

When 零 means 'wither and fall'(凋零), it only refers to flower petals or leaves, and is mostly used in a poetic sense; When 零 means 'drop and shatter' as in (零落), it is mostly used in a poetic sense also. For example: "tears drop and shatter(涕零)"