题干

科学研究表明地球的自转在变慢.四亿年前,地球每年是400天,那时,地球自转的周期为21.5h,科学家们猜想,地球自转变慢的原因主要有两个:一个是潮汐时海水与海岸碰撞、与海底摩擦而使能量变成内能;另一个是由于潮汐的作用,地球把部分自转能量传给了月球,使月球的机械能增加了(不考虑对月球自转的影响),由此可以判断,与四亿年前相比,现在月球绕地球公转的  

A:半径减小

B:周期增大

C:线速度增大

D:角速度增大

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答案(点此获取答案解析)

B

同类题4

Dear Disney,
   Last weekend, the four members of my family spent an enjoyable holiday at the Magic Kingdom - Disneyland. And one event made our visit seem truly magical. We got to the front of the line at Space Mountain, only to find out that our little daughter Gloria couldn't take her ice-cream on the ride. We were so excited that we all forgot ice-cream was not allowed on the ride.

   Gloria broke into tears and we all had no idea what to do. Then Murphy, one of the staff(员工) members, came over. She bent down and told Gloria that she would hold the ice-cream for her and give it to her when she finished the ride. Gloria said, "Promise?" Then she gave Murphy her ice-cream, and she completely enjoyed the ride.
   Sure enough, as we walked out the exit, there was Gloria's new friend with "her" ice-cream. Now you and I both know what happened, because we know that an ice-cream won't last twenty minutes on a summer afternoon in Florida. Murphy knew' what time we would get off the ride; she went to the nearest shop and bought a brand-new ice-cream thirty seconds before we walked out the exit. Gloria said, "Thank you," but I'm sure she thought it was the same ice-cream.
   We know that someone tried hard to make our visit special. Thanks so much for going above and beyond!                                                                                                                                                                                                 Your fan,                                                                                                                          Carmen Rivera