All of the following are responses to external stimuli in the passage EXCEPT - 1.hiding in a tree hole
2.animals returning to their homes
3.the mountain hare's white fur
4.winter's arrival

I think the answer is 4. winters arrival

I just took the test and the answer is actually D) Because Solar cells transform radiant energy from the sun into electrical energy. It is more expensive, however, than the transformation of thermal energy into electrical energy by combustion. Therefore, solar cells would be more practical if they were LESS EXPENSIVE.