Many rivers ________. [ ] A. have became_爱下问搜题

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问题选择题

Many rivers ________. [ ]

A. have became polluted

B. have become polluted

C. have been polluting

D. have become polluting

答案

答案：B

单项选择题

PCB过孔（或导通孔）孔径应大于或等于（），器孔径与板厚之比应不小于1∶3，若更小会引起生产困难与成本提高。

A.0.4mm

B.4mm

C.2mm

D.1mm

填空题

种实带菌方式（）。