Low-carbon lifestyle is of great benefit to improve the world environment．________can be enjoyed from it until you have a deep understanding of it, however．
| A．Something | B．Nothing | C．Few | D．Much |
He paid for a seat, when he ______ have entered free.
| A．could | B．would | C．must | D．need |
— The woman biologist stayed in Africa studying wild animals for 13 years before she returned.
— Oh, dear ! She ______ a lot of difficulties !
| A．may go through | B．might go through |
| C．ought to have gone through | D．must have gone through |
Helen ______ go on the trip with us, but she isn’t quite sure yet.
| A．shall | B．must | C．may | D．can |
The World Wide Web is sometimes jokingly called the World Wide Wait because it ______ be very slow.
| A．should | B．must | C．will | D．can |
— Tom is never late for work. Why is he absent today ?
— Something _______ to him.
| A．must happen | B．should have happened |
| C．could have happened | D．must have happened |