Task: Question Answering
Input (English): Most South American monkeys have color vision. But the members of one genus, Aotus , also known as "owl monkeys," do not have color vision. Scientists think that the genus evolved from monkeys that did have color vision. The ability to see color was lost when it was no longer helpful. Which other trait of the genus would contribute most to making color vision unimportant?  
 (A) The monkeys live in the trees of dense forests. (B) The monkeys make eight different sounds. (C) The monkeys are active only at night. (D) The monkeys choose mates for life.
Output (English): C