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 (燃烧)，is necessary to drive a gasoline engine. A sufficient cause is one which can produce an effect unaided (独自的), 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 (行人) or other cars in the intersection-must also be present.

5. This passage mainly discusses ____.

A) causal reasoning

B) classification of cause

C) various types of reasoning

D) the causal process

注：主题。文中只提到因果关系推理的几种操作方法

Another common type of reasoning is the search for causes and results. We want to know whether cigarettes really do cause lung cancer, what cause malnutrition（营养不良）, the decay of cities, or the decay of teeth. We are equally interested in effects: what is the effect of sulphur or lead in the atmosphere, of oil spills and raw sewage (污水、污物）in rivers and the sea, of staying up late on the night before an examination?