题干

如图所示,皮带传动装置,右轮的半径为ra是它边缘上的一点;左侧是一套轮轴,大轮的半径为4r,,小轮的半径为2rb点在小轮上,到小轮中心的距离为r,已知c点和d点分别位于小轮和大轮的边缘上,若在转动过程中皮带不打滑,则(    )

A:a点和c点周期相等

B:a点和b点角速度相等

C:a点和c点线速度大小相等

D:a点和d点的向心加速度大小相等

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C,D

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    A new study of 8,000 young people in the journal of Health and Social Behavior shows that although love can make adults live healthily and happily,it is a bad thing for young people. Puppy love(早恋)may bring stress for young people and can lead to depression. The study shows that girls become more depressed than boys, and younger girls are the worst of all.

    The possible reason for the connection between love and higher risk of depression for girls is “loss of self”. According to the study, even though boys would say “lose themselves in a romantic relationship”, this “loss of self” is much more likely to lead to depression when it happens to girls. Young girls who have romantic relationships usually like hiding their feelings and opinions. They won't tell that to their parents.

    Dr Marianm Kaufman, an expert on young people problems, says 15% to 20% young people will have depression during their growing. Trying romance often causes the depression. She advises kids not to jump into romance too early. During growing up, it is important for young people to build strong friendships and a strong sense of self. She also suggests the parents should encourage their kids to keep close to their friends, attend more interesting school activities and spend enough time with family.

    Parents should watch for signs of depression—eating or mood changes—and if they see signs from their daughters or sons,they need to give help. The good news is that the connection between romance and depression seems to become weak with age. Love will always make us feel young, but only maturity(成熟) gives us a chance to avoid its bad side effects.