It was not until he saw his mom _________.

A．did the little child stop crying

B．that the little child stopped crying

C．the little child stopped crying

D．when the little child stopped crying

手持离心式转速表测圆周速度时，应使用（）。

A.圆摩擦头，从表上读取的读数要经换算

B.圆锥摩擦头，从表上读取的读数要经换算

C.圆摩擦头，可直接从表上读取圆周速度的读数

D.圆锥摩擦头，可直接从表上读取圆周速度的读数