– Why did they help Mrs Brown with her work?

– Because she was _____.

A．an old friend of them

B．the friend of them

C．on the their friend

D．a good friend of theirs

关于水准式水平仪的工作原理，下列说法中错误的是（）。

A.水准器是一个密封的圆柱形玻璃管，里面装满馏乙醚或精馏乙醚

B.水准器的气泡总是趋向于玻璃管圆弧面的最高位置

C.水准器相对于水平面倾斜越大，气泡的偏移量越大