27. The process of producing electricity may also give out CO2, _______ you heat your house with electricity instead of charcoal.
| A．even if | B．as long as | C．as if | D．as soon as |
I like these English songs and they __________ many times on the radio.
| A．taught | B．have taught |
| C．are taught | D．have been taught |
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 |
He __________ football regularly for many years when he was young.
| A．was playing | B．played |
| C．has played | D．had played |
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 |
—What is the price of petrol these days?
—Oh, it __________ sharply since last month.
| A．is raised | B．has risen |
| C．has arisen | D．is increased |