He has poor eyesight _______computer work.[ ]

A. because too many

B. because too many

C. because of too much  

D. because of much too

原长为16cm的轻质弹簧，当甲、乙两人同时用100N的力在两端反向拉时，弹簧长度变为18cm；若将弹簧一端固定在墙上，另一端由甲一人用200N的力拉，这时弹簧的长度为_________cm，此弹簧的劲度系数为_________N/m。