【单选题】

Questions 5 to 7 are based on the following conversation.At the end of the conversation, you will be given 15 seconds to answer the questions.

Now, listen to the conversation.
As to the experiments taken at the MayoClinic, which of the following statements is trueA．Each volunteer took the same amount of food with about 1000 calories.
B．Volunteers had to wear special devices to record their weight.
C．It turned out that those who had more walking up and down stairs gained much less weight than others.
D．In the experiment every volunteer gained weight.