He is so busy with the research _____ he has no time left for any other activity.

Please make a(n) _______ about your favorite festival and fill out the form.

Einstein’s most famous theory is ______ we call theTheory of Relativity.

Now many people move into cities in order to have their children___.

Good books are like wise friends, because _____ support you to walk forward and help you understand the world.