When did you arrive school this morning?
A:in
B:at
C:to
shèng kāi
wán shuǎ
diào yú
guān chá
____
hé lǒng
yì běn zhèng jīng
yǒu qù
yǐn rén zhù mù
在设计对照实验时,我们应注意的问题是( )