A:H2O
B:H2
C:H2SO4
D:2H
A. What's the matter?
B. It's red and white.
C. Yes, it is! Thank you.
A: ____
B: I can't find my ball.
A: What colour is your ball?
B: ____
A: Look! Is this your ball?
废金属的危害①____②____.
回收利用废金属的优点有____、____.