You must make sure your mask (口罩) is clean ________ you put it on.
在你戴上口罩之前，必须确保它是干净的。
考查连词辨析。before在……之前；after在……之后；since自从；where在……地方。根据You must make sure your mask is clean及you put it on，可知，是在戴口罩前确保它是干净得，故选A。
You must make sure your mask (口罩) is clean ________ you put it on.
_________he was poor himself, he tried his best to help the people in need.
______you smile at others, they will smile back.
—Michael, shall we go hiking in the mountains next Sunday?
—OK, ________it rains heavily.
The young men all shouted “Happy New Year” _______it was twelve o’clock.
You’d better tidy up your room ________ your father comes back.
A break between classes is very important for students because it ________helps with students' health by doing sports _______improves their social skills by communicating with their classmates.
You'd better make a good plan you take a holiday.
Cooking is an interesting lesson and we can make different dishes ________ we like.
—Would you like to play table tennis with me this afternoon?
—I'd like to. ________ I'm afraid I have no time.
