—What areyou busy with — We are carrying out a research ____ thecauses of cancer.

He came home late. You _____him do that, he _______. [ ]

We’ll go for a walk if it tomorrow.

I have never been to the Great wall. —If you have an opportunity _______ there, you must take it.

If I hadn’t stood under the ladder to catch you when you fell, you ____ now.