计算：（﹣6）+（+7）﹣（+2）﹣（﹣1）
解：原式=﹣6+7﹣2+1=1﹣2+1=0．
William spends too much time in Internet cafes, ________he either plays games or chats on the internet.
A．that
B．where
C．which
D．whose
驾驶舱门平衡板有什么作用？
