钻机绞车的带式刹车机构采用双杠杆较单杠杆（）
A.更费力
B.安全性降低
C.更省力和安全
D.操纵难度增大
参考答案：C
The government officials met the workers and engineers working on the stadium, most____ were migrant workers．
A．of which
B．of who
C．of whom
D．of them
自然电位和自然伽马曲线都可以用来判断岩性，其中自然电位还可以确定岩层的泥质含量。（）
