题目内容

电火花线切割加工不能加工不通孔类零件表面。

A. 对
B. 错

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激光加工是利用能量密度非常高的单色光，通过聚焦成平行度很高的微细光束，可得到极大的能量密度和高温以达到去除材料的目的。

A. 对
B. 错

超声波加工，不能加工硬脆材料。

A. 对
B. 错

______into use in April 2000，the hotline was meant for residents reporting water and heating supply breakdowns.

A. Put
B. Putting
C. Having put
D. Being put

—Tom is never late for work. Why is he absent today?—Something _______to him.

A. must happen
B. should have happened
C. could have happened
D. must have happened