hundred只有当用在hundreds of sth (上百个什么什么）里时才用复数形式,其他时候都用单数,哪怕是两百,三百,五百这样容易被误认为需要用复数的情况.
另外,thousand,million等等也是这样
5 hundred people,8 hundred students.
3 thousand; 6 thousand;
thousands of people,millions of population