If nothing________，the oceans will turn into fish deserts.
| A．does | B．had been done |
| C．will do | D．is done |
Though we don’t know what was discussed, yet we can feel the topic_____.
| A．had changed | B．will change | C．was changed | D．has been changed |
By the time he realizes he ____into a trap, it’ll be too late for him to do anything about it.
| A．walks | B．walked | C．has walked | D．had walked |
—Have you known Dr. Jackson for a long time?
—Yes, since she________ the Chinese Society.
| A．has joined | B．joins | C．had joined | D．joined |
--- I’m sure Andrew will win the first prize in the final.
--- I think so. He ___________ for it for months
| A．is preparing | B．was preparing |
C．had been preparing | D．has been preparing |
So far this year we ______ a fall in house prices by between 5 and 10 percent.
| A．saw | B．see | C．had seen | D．have seen |