单选题 The aging process may not be the result of a rigid genetic program that in itself dictates longevity. On the contrary, what we see as maximum life span may simply be the complex and indirect result of multiple traits in the organism that are internally tied to normal development. In other words, it is not that the body is somehow pre—programmed to acquire gray hair, wrinkles, or diminished metabolic (新陈代谢的) functions. Rather, these signs of aging are simply telltale side effects of activities of the organism.
Consider the analogy of an "aging car". Suppose a distinctive "species" of automobile were designed to burn fuel at a fixed temperature with an efficient rate of combustion (燃烧). That specific rate of combustion is required for appropriate acceleration, cruising speed, fuel mileage (油耗), and so on. But, when the car functions in this way over a period of time, the car also, of necessity, produces certain emission by—products that, over time, begin to clog the cylinders, reduce automotive efficiency, and lead to the breakdown and final collapse of the machine.
In the case of the human "car", it could be the burning oxygen in normal metabolism generates harmful by—products in free radicals that prove toxic to the organism. What we see here may be a basic trade—off: oxygen is essential for life yet harmful to our eventual well—being. In this view, the human "car" is not intentionally designed to accumulate toxic emissions in order to collapse. But there seems to be no way for the car to function at optimum levels without the destructive by—products.
But suppose we could find some special "fuel additive" that eliminates toxic emissions. Would we then have an "immortal" car? Probably not. Changing the fuel used in your car won"t prevent accidents, nor would any fuel additive prevent rusting or the wearing down of springs and shock absorbers. The human "car" analogy, of course, is misleading, because an organism, unlike a manufactured object, has a capacity for repair and self—generation, at least up to certain point. The whole question about why we grow old is finding out why that capacity for self—repair ultimately seems unable to keep up with the damage rate: in short, why aging and death seem to be universal.

单选题 From the passage, we learn that ______.

单选题 The example of the "aging car" is used to make the point that ______.

单选题 A "basic trade—off" (Line 3, ParA. 3) is a process ______.

单选题 Which of the following statements is true according to the passage?

单选题 The "human car" analogy is faulty in the way that ______.