Causes are classified as necessary, sufficient, or contributory(起辅助作用的). A necessary cause is one which must be present for the effect to occur, as combustion ，is necessary to drive a gasoline engine. A sufficient cause is one which can produce an effect unaided , though there may be more than one sufficient cause: a dead battery is enough to keep a car from starting, but faulty spark plugs(火花塞)or an empty gas tank will have the same effect. A contributory cause is one which helps to produce an effect but cannot do so by itself, as running through a red light may help cause an accident, though other factors --- pedestrians or other cars in the intersection-must also be present.
4. Your refrigerator is not working and you have found that the electric power has been cut off.
The power failure is a ____.
A) necessary cause B) sufficient cause
C) contributory cause D) none of them
注：断电不是冰箱不工作的.必要原因，而是原因之一，冰箱坏了也可以导致冰箱不工作。
Causes are classified as necessary, sufficient, or contributory(起辅助作用的). A necessary cause is one which must be present for the effect to occur, as combustion ，is necessary to drive a gasoline engine. A sufficient cause is one which can produce an effect unaided , though there may be more than one sufficient cause: a dead battery is enough to keep a car from starting, but faulty spark plugs(火花塞)or an empty gas tank will have the same effect. A contributory cause is one which helps to produce an effect but cannot do so by itself, as running through a red light may help cause an accident, though other factors --- pedestrians or other cars in the intersection-must also be present.
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