什么是水平移动系数？
参考答案：
在充分采动或接近充分采动的条件下，沉煤层走向的最大水平移动与地表最大下沉之比。
Mr Smith __________in London for a few years before he _____to China.
A．lives; came
B．has lived; came
C．had lived; comes
D．had lived; came
