-- _____ classes are there in your school?   –Thirty. 

---Why would some workers in Foxcon die __________ continue working in the factory?  ---Psychologists say they are under too much pressure. 

Frank often wears a mask _____ he can keep fit in the foggy and hazy (霾) weather.

_________ we know, food gives us energy.

—Is there a park near here?            —No, there isn’t. _____there is a post office.