from the first dynasty-the Xia Dynasty,which was started from 21 century B.C till the Jin dynasty ,over 200 kings had the capitals here,and have left numberous historical relics remain underground.这句话里,为什么which was started,要加was?这个was起到什么作用?不加的话,直接说which started可以不?为什么,请高手作答,谢谢。