分离下列各组物质一定会用到过滤操作的是（ ）
A．水和汽油
B．蔗糖和水
C．氧气和氮气
D．氯酸钾和二氧化锰
答案：D
— You see, I'm poor in maths, you are not good at English.
— That's ________ we should help each other. [ ]
A. when
B. where
C. how
D. which
