Helen and Bob are working hard these days because they will put ______ a short play in the school Talent(才艺) show.
A：over
B：away
C：off
D：on
It's impolite ____information directly when asking for help.
打雷闪电的时候，空气中有极少的氧气（ O2）变为臭氧（O3），下列说法正确的是（ ）
台湾民族以汉族为主，少数民族主要是（ ）
