Whether you like it or not, we live in a particular community_______ exchanging presents is an important part of communication.

A．which

B．that

C．when

D．where

如图所示，在倾角为α的斜面上，重为G的小球被竖直的木板挡住，不计一切摩擦，则小球对斜面的压力为____________，小球对木板的压力为____________。