_____ the chapter four times, I finally understood the author's theory．
| A．Reading | B．Having read | C．To read | D．Read |
It is well understood _____ controls the flow of carbon dioxide in and out the atmosphere today.
| A．that | B．when | C．what | D．how |
–How far apart do they live?
–_____ I know, they live in the same neighborhood.
| A．As long as | B．As far as | C．As well as | D．As often as |
He was about to tell me the secret _____ someone patted him on the shoulder.
| A．as | B．until | C．while | D．when |
He’s got himself into a dangerous situation _____ he is likely to lose control over the plane.
| A．where | B．which | C．while | D．why |
I have waited for her since _____.
| A．two hours ago | B．two hours before |
| C．two hours | D．two hour’s time |