阅读理解。 You know the sound of your heartbeat: lub-dub, lub-dub. In some people, there's an extra noise that the blood makes as it flows through the heart. It sounds sort of like the noise of flowing water. This sound is called a murmur. Just like kids, murmurs have grades. Grade l is the softest-sounding murmur, and Grade 6 is the loudest. A murmur graded 4, 5, or 6 is so loud you can actually feel a rumbling from it under the skin if you put your hand on the person's chest. Most murmurs don't mean anything is wrong. Doctors may call these 'innocent', 'functional', or 'normal' murmurs. One common type of normal murmur is Still's murmur, named for the doctor who first described it. This murmur is most often heard in healthy children aged 3 to 7. A normal murmur can get louder when the blood flows faster through the heart, like when kids have a fever or run around. That's because an increase in body temperature or activity makes the heart pump more blood. When your temperature goes down, the murmur may get quieter or even disappear. It can be easier to hear heart murmurs in kids because they have less fat, muscle, and bone between the murmur and the doctor's stethoscope (听诊器). Many normal murmurs become harder to hear as children grow older, and some eventually disappear. Even though most murmurs do not mean anything is wrong, sometimes a heart problem can cause a murmur. If your doctor thinks your heart murmur could be due to a heart problem, you will need to see a pediatric cardiologist. This kind of doctor knows a lot about children's hearts. 1. What's the text mainly about? A. Heart problems. B. Heart murmurs. C. Murmur types. D. Murmur grades. 2. We can hear a kid's louder heart murmur when he has just _____. A. woken up from a sound sleep B. listened to some light music C. played football for two hours D. finished reading a storybook 3. This text is most probably taken from _____. A. a science magazine B. an experiment report C. a hospital introduction D. a medical advertisement 4. We can learn from the last paragraph that _____. A. most heart murmurs do harm to kids' health B. some heart murmurs can cause heart problems C. doctors know what has led to low temperatures D. a certain heart problem can cause a heart murmur