The child with rheumatic fever must have his heart rate measured while awake and while sleeping. Why are two readings necessary ？（）
A．To obtain a heart rate that isn't affected by medication.
B．To eliminate interference from the jerky movements of chorea.
C．To ensure that the child can't consciously raise or lower the heart rate.
D．To compensate for the effects of activity on the heart rate.
参考答案：D
解析：
Tachycardia may be a sign of heart failure. Mild tachycardia is more easily detected during sleep than during the day when activity can cause an increase in heart rate. Medications given for rheumatic fever and rheumatic heart disease, such as digoxin, exert their influence day and night. Chorea, a symptom of rheumatic fever, is the loss of voluntary muscle control. However, it doesn't affect pulse because the child would be sitting quietly and not involved in purposeful movement. A ten-year-old child is unlikely to be able to consciously raise or lower his heart rate.