On Sundays there were a lot of children playing in the park, ________ parents seated together joking.
— Remember the first time we met, Jim? — Of course I do. You _______ in the library.
I always prefer _______ a bike rather than _______ to work.
When you read the book, you’d better make a mark _____ you have any questions.
AIDS is said _______ the biggest health challenge to both men and women in that area over the past few years.
________ the problem of land erosion(侵蚀) getting more serious, the government is searching for a way to deal with it.