It disappointed his parents___he had failed to pass the exam for ______third time.

A．because ; a

B．that; a

C．when; the

D．that; the

一根轻质弹簧，当它上端固定、下端悬挂重为G的物体时，长度为L₁；当它下端固定在水平地面上、上端压一重为G的物体时，其长度为L₂．两种状态下弹簧均未超过弹性限度，则该轻质弹簧的劲度系数是（　　）

A．

B．

C．

D．