若平面任意力系向某点简化后主矩为零,则其合力()。
A.一定为零
B.一定不为零
C.与主矩相等
D.不一定为零
参考答案:D
The air in Beijing is getting much ______ now than a few years ago.
A.clean
B.cleaner
C.cleanest
D.the cleanest
阻塞性肺气肿患者肺通气指标肯定下陴的是
A.1秒用力呼气量/用力肺活量B.肺活量C.潮气量D.功能残气量E.肺总量