一个物体做匀速圆周运动，关于其向心加速度的方向，下列说法中正确的是(   )

    Our ancestors（祖先）made the old saying that friends____(be) like wine-the older,the better.Because of that, many people think old friends to be____(important) than new ones.For example,if two pieces of advice are given to solve a problem,one from a new friend and the other from an old friend,people always prefer to choose the latter one,____the new friend's advice may be better.

    ____(disagree) with the old saying,I believe that new friends are not____(necessary) worse than old friends. Why?Because the____(long) of time cannot tell whether your friendship is better or not.

    Once you call someone friend,he must be a person who is____(rely) and whose interests are in common with____(you).Maybe a friend's outside look____(change) all the time,but the inside characters of him and his interests won't change at all.It is just these unchangeable characters and interests that make him a friend to you.____,there is no difference between old friends and new ones on this point.

火车站台边缘处标有一条安全线，人必须站在安全线以外的位置候车。其原因是火车急速驶过车站时，安全线与列车间的空气流速____ ，压强____ （均选填“大”或“小”），易发生安全事故。