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