Read the following paragraph carefully and choose the best topic sentence from the four possible answers that follow the paragraph by writing A, B, C or D. 
Researchers divided the 1,400 subjects into two groups. The first group did not stretch before their runs. The second group stretched, having received photographs and specific instructions for a series of simple, traditional poses, like leaning over and touching toes. The volunteers followed their assigned regimens (方法) for three months. About 16 percent of the group that did not stretch were hobbled (跛行) badly enough to miss training for at least three days (the researchers’ definition of a running injury), while about 16 percent of the group that did stretch were laid up (生病) for the same amount of time. The percentages, in other words, were virtually identical.