已知自然界氧的同位素有16O、17O、18O,氢的同位素有H、D,从水分子的原子组成来看,自然界的水一共有[ ]
A. 3种
B. 6种
C. 9种
D. 12种
答案:C
屈原在作品中主要表达什么样的思想感情?
You _____ be hungry already-you had lunch only two hours ago! [ ]
A. wouldn't
B. can't
C. mustn't
D. needn't