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如图,一内壁光滑的圆锥面,顶点O在下方,锥角为2α,OO′是竖直轴线,若有两个相同的小珠(均视为质点)在圆锥的内壁上沿不同的圆轨道运动,则(   )

A:它们的向心力之比等于半径之比

B:它们的周期之比等于半径之比

C:它们的动能之比等于半径之比

D:设O点为势能零点,它们的动能之比等于重力势能之比

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C,D

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The Advantages of Hobbies

    Hobbies provide entertainment and a chance lo continuously learn something new and develop a skill set. ____

    Hobbies are praised as a key to stress reduction.Becoming lost in the mental concentration or physical action which you are deeply interested in helps you shift focus from the parts of life that are stressful. ____ And in turn you are more peaceful when working on other work.

    ____ Many people set goals with their hobbies that provide them satisfaction when these goals have been achieved.These goals can be simple,like improving a golf score,writing a certain number of pages for a novel each day or seeing an entire collection of classic films.

    Hobbies provide a sense of community when you meet other people who enjoy similar activities. ____ More populated cities typically have numerous clubs or groups for people to share their knowledge and enthusiasm about a particular hobby.Becoming a member of one of these organizations allows greater growth within your hobby,as well as a chance to make new friends.

    Having a hobby can develop a talent that makes a person feel proud and satisfied.For example,a tennis hobby developed at a young age can lead lo a career as a professional athlete one day. ____For example, writer Vladimir Nabokov also collected butterflies,which allowed him to do better in his chosen fields because he used learned skills from one to improve the other.

A.There are a variety of hobbies.

B.Hobbies provide a sense of achievement.

C.Some hobbies affect other aspects of your life.

D.Focusing on something enjoyable can calm the mind.

E.They also provide many other social and health benefits.

F.Having a hobby increases the chances to interact with like-minded people.

G.Some hobbies containing physical activities offer an enjoyable way to stay healthy.