The central idea of this passage is that______.A．student performance improves, but at a hi

The central idea of this passage is that______.

A．student performance improves, but at a high cost

B．it"s silly to improve achievement by extending school days

C．lengthening the school day is impossible at most schools

D．school time should be lengthened at any cost

请帮忙给出正确答案和分析，谢谢！