电机（）周围保持良好的通风，以保证电机（）的正常运转。

The boys are _________.[ ]

A. taken good care of  

B. taken good  

C. taken good care      

D. taken care

有一块菜地为梯形，上底是13米，比下底短8米，高是50米，去年共收白菜11900千克，平均每平方米收白菜多少千克？

Months ago we sailed ten thousand miles across this open sea, which ________ the Pacific,

and we met no storms. [ ]

A. was called  

B. is called

C. had better called  

D. has been called

临床常用的根管消毒方法是

A．电解治疗
B．根管封药
C．根管荡洗
D．微波处理
E．高频电疗

破乳剂是防止水基压裂液在井眼附近和地层原油发生乳化现象的。（）