You never know how much difficulty we had _____ the problem. Luckily, we managed to solve it with Mrs Wu’s help
—Does your father still smoke?—No, he has succeeded in_____ smoking for three years.
—How do you like Beijing, Miss Read?—I’ve no idea. I _____ there.
—Where are Maria and Kangkang?—They _____ England.
—Have you found your lost book_____?—No, I haven’t.
The _____ population has caused many serious problems.