_____ he is very busy, he spends at least two hours every day surfing the Internet in order to find out the latest development in science and technology.
| A．Even though | B．When | C．Unless | D．Whether |
By the time he arrived in Beijing, we _______ there for two days.
| A．will have stayed | B．stay | C．have been staying | D．had stayed |
The professor could hardly find sufficient grounds________ his arguments in favor of the new theory.
| A．on which to base | B．which to base on | C．to base on | D．to be based on |
Along with the email was my brother’s promise ______ I came first in the competition, he would buy me an MP4.
| A．what | B．that | C．that if | D．whether |
"We can’t go out in this weather," said Bob, ________ out of the window.
| A．looking | B．to look | C．looked | D．having looked |
Scientists are convinced _______the positive effect of laughter ______ physical and mental health.
| A．of ; at | B．by ; in | C．of ; on | D．on ; at |