Help others whenever you can _______ you will earn respect from others without difficulty.
| A．and | B．or | C．unless | D．but |
—Why does the lake smell terrible ?
--because large quantities of water ____.
| A．have polluted | B．is being polluted | C．has been polluted | D．have been polluted |
My parents have promised to come to see me before I ___for Africa.
| A．have left | B．leave | C．left | D．will leave |
The way the guests____in the hotel influenced their evaluation of the service.
| A．treated | B．were treated | C．would treat | D．would be treated |
Over the past decades , sea ice _____in the Arctic as a result of global warming.
| A．had decreased | B．decreased | C．has been decreasing | D．is decreasing |
--I would why he ____so strangely these days.
--Recent pressure at study may account for his behavior.
| A．was acting | B．is acting | C．acted | D．acts |