Why Did Nathaniel Kleitman and His Students and Other Volunteers

Why did Nathaniel Kleitman and his students and other volunteers spend over a month in a cave?

A) He wanted to see if people could adapt to a 28-hour day.
B) He wanted to see how the lack of natural light would affect levels of depression.
C) He was interested in how the cold in the cave would impact the functioning of the immune system.
D) He was interested in how groups would form when everyone was in a potentially dangerous situation.