The two-stroke cycle begins with the piston coming up from the BDC, with the scavenging ports in the sides of the cylinders being （）.

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• The four-stroke cycle begins with the piston （）.

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• 听力原文： Each night we alternate between two kinds of sleep: active sleep and passive sleep. The passive sleep gives our body the rest that is needed, and prepares us for active sleep, in which dreaming takes place. And we arc beginning to understand the importance of the dream period in our sleep cycles.

  In passive sleep the body is at rest. We move very little. And the brain becomes very inactive, ff a person continues to sleep, she or he enters anew period, a more active period. The body goes through several changes: the brain temperature rises, blood in the brain increases, the body becomes very still and the brain goes into active state. And as the brain goes active, the eyes begin to move rapidly. And that is the sign of another change---the person is dreaming. Throughout the night, people alternate between passive and active sleep. Doctors have studied the sleep cycle and have found that everyone dreams---in fact, that everyone needs to dream in order to stay healthy. It appears we need both kind of sleep. We need passive sleep in order to rest our bodies. We need active sleep in order to dream. And dreaming helps us to rest our minds, What does not happen in the brain in the active sleep？ A．The brain becomes very active. B．The brain becomes very inactive. C．The brain temperature rises. D．The brain blood rises.

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