Not until his mother came back_______.
| A．he began to do his homework | B．had he began to do his homework |
| C．he had begun to do his homework | D．did he begin to do his homework |
Expected noises are usually more _____ than unexpected ones of the like magnitude.
| A．manageable | B．controllable | C．tolerable | D．perceivable |
Experts say walking is one of the best ways for a person to _____ healthy.
| A．preserve | B．stay | C．maintain | D．reserve |
Jack almost fell off the cliff, but managed to _____ until help came.
| A．keep on | B．catch on | C．count on | D．hang on |
The storm sweeping over this area now is sure to cause _____ of vegetables in the coming days.
| A．rarity | B．scarcity | C．invalidity | D．variety |
Don't _____ the news to the public until we give you the go-ahead (许可，准许).
| A．release | B．discard | C．relieve | D．retain |