In the past few years, a great many teachers have found       very important to teach their students how to learn.

—Which of these two sports sweaters will you take?—I’ll take           . They look nearly the same, and I just need one.

Please keep______ secret that he’ s decided to give up his present position.

Many Chinese students found______ difficult to learn English grammar.

I had to buy ________ these books because I didn't know which one was the best.