The two researchers originally set out to study the zebras in Africa and discovered a new species of tiger ______.
| A．by chance | B．in addition |
| C．by nature | D．in need |
--- Tom , why didn’t you answer my call yesterday afternoon?
--- Sorry , I didn't hear the phone . I ___________ asleep.
| A．must be | B．must have been |
| C．could be | D．should have been |
If you want to change for a red one you'll have to pay ___________ $ 20.
| A．another | B．other | C．more | D．each |
September 18, 1937 is a day _______ all Chinese will remember forever.
| A．when | B．what | C．不填 | D．why |
There are some health problems that , when _______ in time , can become bigger ones later on.
| A．not treated | B．not having treated |
| C．not to be treated | D．not having been treated |
You will be surprised how quickly you _____ the money , so it is worthwhile to make a plan in advance.
| A．put away | B．get through | C．run out | D．end up |