单项选择题

如果某产品的销售额仅在每一年的9月、10月和11月有明显的月度变动，为了研究产品销售额的月度效应，在一个有截距的回归模型里，需要引入几个虚拟变量（）。

A、1
B、2
C、3
D、4

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听力原文：Researchers have found that daily walking may improve sleep quality. They have also noticed that only those who exercise in the morning have the beneficial effects on sleep. Those who exercised in the evening actually had more trouble falling asleep. One possible explanation is that morning versus evening exercise may affect sleep quality. Morning exercise may get the body clock in good order, and evening exercise may upset it. However, more research is needed to confirm this theory.
(57)

答案： A.

单项选择题

表示带受电弓的动车（）

A.Tc
B．Mp
C．M
D．“—”