在一个盒子里装有均匀的已编有不同号码的五个白球和七个黑球，则：从中任取两球都是黑球的

Directions: Read the following text and choose the most suitable heading from the list A-F for each paragraph. There is one extra heading which you do not need.

A. The engine in your body. B. The location, size and heartbeat of a heart. C. What happens when the heart beats? D. How does your heart work? E. How do we know about the heart? F. What can a doctor tell by feeling your pulse?

小题1:
Your heart is located in your chest, a little to your left. This heart of yours, which is about the size of your two fists held together, beats about 90 times a minute. A grown person's heart beats about 60 to 80 times a minute. The heartbeat is not just the same in all persons, and it is not the same in any one person at all times.
小题2:  
When your heart beats, it is pumping blood to all parts of your body. If you could examine your heart closely, you would see that it is really two pumps placed side by side, and working at the same time. Each pump has two parts, the upper part called the auricle (心房), and the lower part called the ventricle (心室). The auricles receive the blood as it comes into them after it has been pumped through the body. The ventricles pump the blood out. The right one pumps the blood to the lungs and the left one pumps the blood to all other parts of the body. At the top and bottom openings of each ventricle are valves (阀门) which make the blood go in only one direction.
小题3:
Your heart is sometimes called the engine or the motor in your body and sometimes called the pump. It works away, both day and night. First it pumps out some blood, rests for a few seconds, and then it pumps some more. In a normal day, the heart pumps about 2,500 gallons of blood from the auricles and ventricles.
小题4:
By using a stethoscope to listen to the heart, the doctor can tell whether your heart is beating evenly and whether the valves are closing tightly. The stethoscope makes these sounds so clear that the doctor can hear them easily. The stethoscope has an earpiece that he places on your chest and tubes that he places in his ear. The earpiece carries the sound or your heart's beating along the tubes to the doctor's ears, and it makes the sound seem much louder than it really is. The doctor could listen to your heartbeat by pulling his ear against your chest.
小题5:
An easy experiment can help you understand what happens when the heart beats. You can do this experiment with a hollow rubber ball. Make a small hole in it, and fill the ball with water through the hole. When you squeeze the ball, you will notice how the water comes out in a spurt each time you squeeze. After each spurt the ball comes back to its round shape again. Something like this happens when your heart beats. The muscles in your heart grow smaller, or contract, and squeeze the blood out of the heart. Each time this happens, we say your heart is beating. Perhaps you have noticed that the doctor places his finger on the pulse in your wrist when you are ill. By doing this he can tell how fast your heart is beating.