My high school friend,         I haven't seen for 20 years, is no longer ____ he was. [ ]

A. who, that

B. which, that

C. whom, what

D. that, what

在泥泞的道路上空手行走往往比挑适量担子行走   滑得厉害些，这是由于挑担子后对地面的压力___________ ，从而使摩擦力___________的缘故。