样品在机械轨道上转运，通常一组的试管数是（）。
A.1~2个
B.1~4个
C.4~5个
D.4~10个
E.4~20个
参考答案：D
—May I come in! I’m sorry I am late.
—Come in, please tell me .
A、why you are late again
B、what were you doing
C、who did you talk with
烟度计检测时，使发动机加速2－3次，吹净（）与消声器的烟灰。
A、排气门的碳粒
B、排气管
C、排气门处的杂质
D、排气道
