You have been assigned to write a research essay on sleep. Your current working research question is "What is sleep?"

Which revision of this question would be most effective for improving and tightening your research?
Responses
A "Why do people love to sleep so much?""Why do people love to sleep so much?"
B "Do most people sleep in beds or sofas?""Do most people sleep in beds or sofas?"
C "How many hours of sleep do most people get?""How many hours of sleep do most people get?"
D "In what ways is sleep necessary to our health?""In what ways is sleep necessary to our health?"