请计算出NH4NO3的相对分子质量．
NH4NO3的相对分子质量为：14×2+4+16×3=80．
—You can go to the classroom and ask Mary for help.
—She _______ be in the classroom. I have just seen her on the playground. [ ]
A. must
B. mustn't
C. can't
D. may
苏轼写《定风波·莫听穿林打叶声》时年龄是（）
A、30
B、35
C、40
D、45
