单项选择，从A、B、C中选出最佳的答案。
1）— SARS（非典）is such a terrible disease（绝症）.
— Yes, it is. We _____ be more careful.
A. can B. may C. must
2） I have my own（自己的） room in my house, so I _____ do whatever（无论什么） I want in it.
A. must B. have to C. can
3） — Hey, someone says people can live（活到） for 500 years.
— I don't think they _____. Is today April Fool's Day（愚人节）?
A. can B. can't C. must
4） — Why are you working so quietly（安静地）?
— Because the baby _____.
A. is sleeping B. sleeps C. sleep