根据句意用括号中所给词的适当形式填空：
1.One of his __________(foot) hurts.He has to go to see the doctor.
2.Pork is __________(popular) than beef in China.
3.They are talking to each other __________(happy).
4.Thank you for __________(teach) month of the year.
5.Don't worry.I can after _______(I).
顺便说明原因