单阀缓解机车时可不受（）的限制。
参考答案：自阀
A．相对静止
B．向下做匀速直线运动
C．向下做匀加速直线运动
D．向下做自由落体运动
Last week, I bought an interesting story book, _________ is very beautiful.
A．which cover
B．the cover of which
C．its cover
D．the cover of whose
