一道M模考逻辑题~~~ 不懂 求各位大神指教

tantan08

Scientists have discovered a new species of butterfly that lives only in a small region of Central America and is active only at night. During the day, it rests in treetops, where its green color matches the foliage perfectly. Therefore, the scientists must have discovered the butterfly at night.

The argument depends on which of the following assumptions?

A.The foliage in the butterfly's habitat is completely green.

B.There is no way for the scientists to detect the butterfly during the day.  

这两个到底应该选哪一个呢，题目里不是 where its green color matches the foliage perfectly   为什么不能选A选项呢
求指教~~~~~~~~   thanks~~~~~~~~~

tantan08

自己顶一下~~~