It is clear that some people who participate in exercise training will develop injuries to their bones, muscles, and joints (关节). Despite unfounded reports in the mass media of extremely high injury rates among adult exercisers, there have been few good studies of exercise injuries in populations. One of the difficulties in performing such studies has been the need to identify both the number of cases and the number of people at risk for injury. In other words, it is difficult to know the number of individuals injured and the total number of individuals exercising in the population. But these two figures are necessary in order to calculate (统计) tree injury rates. Normally, injury is defined as an accident that causes a person to stop exercising for at least one week. The best available studies on injury rates show that about 25 to 30 percent of adult runners become "injured" (based on the above definition) over the course of a year. More serious injuries include those for which the injured person seeks medical care. If only they are considered, injury rates are much lower, perhaps in the range of I percent per year.
Little is known about the causes of exercise injuries. One factor that has been linked to injury is the amount of exercise; for example, individuals who run more miles are likelier to be injured than those who run fewer miles. Factors such as age, sex, body type, and experience have not been shown to be associated with risk of injury. It seems logical that structural abnormalities, sudden increases in training intensity, and types of equipment used are likely to be related to injury risk. However, data to support these opinions are not available.
The true injury rates are obtained from（）
A．the average number of injuries for each individual
B．the amount of exercises for each individual
C．the numbers of the injured and all individuals in training
D．the numbers of people doing exercises and those stopped exercises for one week
