在循环管路中安装回水（）并定期进行排放，可有效避免循环管路堵塞。
A、加热器
B、旁路管
C、除污器
D、排水阀
参考答案：C
Zhao Xin couldn't go on working the heavy rain the day before
yesterday. [ ]
A. in
B. because of
C. because
D. since
手榴弹
