求英语单词后面的“s"读音清浊的规律?

清发清,浊发浊,元音后面也发浊.清辅音结尾的,S发清音,浊辅音结尾的,S发浊音,元音后面也发浊音.iz的发音是es才会有.
单独看你这个题的话,s的有speaks,laughs,gets,helps.z的有enjoys,tomatoes,studies,means,finds,gives.iz的有brushes,glasses,changes,washes.