高频收发机和以下哪个系统没有交连（）
A.REU
B.PSEU
C.SECAL
D.VHF
参考答案：D
I'm certain Davel's told you his business troubles. _____, it's no secret that he owes a lot of money to
the bank.[ ]
A. However
B. Anyway
C. Therefore
D. Though
什么作为管网末端水中没有污染的信号（）
A.总大肠菌群
B.大肠菌群
C.游离性余氯
D.水的硬度
E.细菌总数
