20．We met _____ and have become good friends since then.
| A．by mistake | B．by accident | C．by ourselves | D．by the way |
______ it have been the failure to pick up directions from the ground control that caused the air crash?
| A．Must | B．Could | C．Would | D．Should |
---What’s the matter with Christine?
---A letter from her parents in California_____an attack of home sickness.
| A．took off | B．set off | C．gave off | D．sent off |
After the quarrel, I tried all my best to fix the misunderstanding between us, but all my efforts seemed _____.
| A．in trouble | B．in case | C．in effect | D．in vain |
Failure hurts grown-ups and children_____, but it can make a positive contribution to your life once you learn to use it.
| A．actively | B．likely | C．alike | D．similarly |
If the food tastes nice, I’ll go to buy some more; _____, I won’t go.
| A．if not | B．when not | C．if any | D．when ever |