The purpose of the three－year study is to______.A．find out why some children find it diffi

The purpose of the three-year study is to______.

A．find out why some children find it difficult to go to sleep

B．learn more about the link between sleep and weight

C．identify the ways parents reduce their kids" weight

D．see if there is difference in sleep patterns over the period

请帮忙给出正确答案和分析，谢谢！