求英语单词后面的“s读音清浊的规律?

求英语单词后面的“s读音清浊的规律?
求归类：
enjoys speaks brushes tomatoes laughs studies means glasses gets finds changes helps washes gives
词尾发/s/音的有（ ）发/z/音的有（ ）发/iz/音的有（ ）

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.