AIDS is said ______the biggest health challenge to both men and women in that area over the past few years.
Only when _________________________.
According to news reports, personal ____ of guns in the USA causes a lot of trouble.
---Did you go to the cinema last night? ---Well, I had ____ to , but it was raining too hard.
Sorry, we are late,--our plane was _____ because of the heavy rain.
I’d like to have visited the Palace Museum, but something unexpected _______ to my father.