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  3. The human heart is enclosed in a sac with a two-layered wall. A lubricating fluid fills the space between the two walls, enablin

The human heart is enclosed in a sac with a two-layered wall. A lubricating fluid fills the space between the two walls, enablin

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问题 The human heart is enclosed in a sac with a two-layered wall. A lubricating fluid fills the space between the two walls, enabling them to slide past each other as the heart pulsates. The heart is mostly cardiac muscle tissue divided into four chambers, two atria and two ventricles. The atria have relatively thin walls and function as collection chambers for blood. The ventricles have thicker walls and are much more powerful than the atria, especially the left ventricle, which must pump blood to all organs of the body. A When the heart beats, the ventricles contract and pump blood. As the heart relaxes after a contraction, the atria fill with blood. B One complete sequence of pumping and filling is called the cardiac cycle. C In an average adult at rest, the cycle takes about 0.8 second. The number of times the heart beats each minute is the heart rate, which can be measured by taking the pulse. D On average, a healthy adult at rest has a heart rate of about 70 beats per minute. An individual’s heart rate will vary, depending on the level of activity.
Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit? The pulse is the rhythmic stretching of the arteries caused by the pressure of the blood when the ventricles contract.
选项 A、 
B、 
C、 
D、 
答案D
解析 The added sentence defines pulse, which develops the phrase taking the pulse, introduced in the previous sentence.
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