单选题 What does this paragraph imply?

A、 Most of us don't get enough exercise.
B、 We should exercise 15 minutes per day.
C、 A high pulse rate is more healthy.
D、 Regular exercise can strengthen our heart.

【正确答案】 D

【答案解析】解析：由文章可知，该实验说明，经常锻炼可以增加心脏功能。