天然水的色度一般是因（）引起.
A.水中悬浮物
B.水中溶解性杂质
C.水中的有机物
D.其它
参考答案：C
单元工程指分部工程中由几个工种施工完成的( )。
A．最小结构
B．最小综合体
C．独立结构
D．独立综合体
Not until he had left his friend _______ how important friendship was for him.
A．did he realize
B．he realized
C．had he realized
D．he had realized
