原水经加（）后后色度大大降低，能去除恶臭，对水中的酚、铁、锰等杂质有非常明显的去除作用。
A、氨
B、氯
C、氢
D、氧
参考答案：B
It was in September ______ we came to NO.3 Middle School.
[ ]
A. that
B. before
C. when
D. since
移民流行病学
