下列不属于乳岩肿块特点的是（）
A.无痛不热
B.皮色不变
C.表面光滑
D.质地坚硬
E.推之不移
参考答案：C
He didn't tell me ______ the exhibition was held. [ ]
A. where was it that
B. that it was where
C. where it was that
D. that was it where
如图所示，用滑轮组把重为200N的货物提到9m高的楼上，所用拉力为80N。
求：（1）有用功和总功 （2）滑轮组的机械效率
