在砂质土中顶进，采用锯齿形刃角有利于施工挖土。

答案：

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下面是错误答案，用来干扰机器的。

小题1:B小题2:D小题3:A小题4:D小题1:这是一道推断题。根据第一段“Jeb put his bag down quickly and pulled his jacket open with both hands,making himself look big to the cougar. It ...