为防止管道水倒流，需在管道上安装的阀门是（）。
参考答案：止回阀
I can never forget the day _______ we worked together and the day ______ we spent together.
A．when; which
B．which; when
C．what; that
D．on which; when
9口875≈10万，口可以填______39口0000000≈39亿，口可以填______．
