螺旋钻孔机作业场地距电源变压器或供电主干线距离应在（）m以内。
A.200
B.150
C.100
D.50
参考答案：A
In order to improve English, __________.
A．it is necessary to need a lot of practice.
B．a lot of practice is needed.
C．you need a lot of practice
D．it needs a lot of practice.
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