According to the author, scientists______.A．have already proved that coffee is good for hu

According to the author, scientists______.

A．have already proved that coffee is good for human health

B．have a long way to go before they find a way to study coffee

C．have avoided the cause-and-effect approach to study coffee

D．are still unable to figure out why coffee is good for us

请帮忙给出正确答案和分析，谢谢！