Fifty volunteers were alphabetically divided into two equal groups, Group A to participate

in a 7 week exercise program, and Group B to avoid deliberate (故意的，蓄意的) exercise of any sort during those 7 weeks. On the day before the exercise program began, all 50 men participated in a step-test. This consisted of stepping up and down on a 16-inch bench at 30 steps a minute for 5 minutes. One minute after completion of the step-test, file pulse rate of each subject was taken and recorded. This served as the pretest for tile experiment. For the next 7 weeks, subjects in the experimental group (Group A) rode an Exercycle (a motor-driven bicycle-type exercise machine) for 15 minutes each day. The exercise schedule called for riders to ride relaxed during the first day's ride, merely holding on to the handle bars and foot pedals as tile machine moved. Then, (or the next 3 days, they rode relaxed for 50 seconds of each minute, and pushed, pulled, anti pedaled actively for 10 seconds of each minute. The ratio of active riding was increased every few clays, so that by the third week it was half of each minute, and by the seventh week the riders were performing 15 solid minutes of active riding.

At the end of the 7 weeks, the step-test was again given to both groups of subjects, and their pulses taken. The post-exercise pulse rates of subjects in the experimental group were found to have decreased an average of 30 heart beats per minute, with the lowest decrease 28 and the highest decrease 46. The pulse rates of subjects in the control group remained the same or changed no more than 4 beats, with an average difference between the initial and final tests of zero.

Tile step-test was given ______.

A．after each exercise period

B．at the beginning and at the end of the seven week period

C．only once, at the beginning of the seven week period

D．twice to the men in Group A and once to the men in Group B