平衡力系中任意一力对于其余的力来说都是平衡力。
参考答案：对
Mom was usually busy ______ my younger sister during the night and she ______ have less sleep.[ ]
A. tending; tended
B. tended; tending to
C. tending; tended to
D. tended to; tending to
焦-片距增大一倍，X线胶片感光量相应()
A．增加1／Z倍
B．增加1／4倍
C．增加1／8倍
D．减少至1／2
E．减少至1／4
