this is a cat变复数是these is.还是they is.

this is a cat变复数是these is.还是they is. Is it a cat?要求变复数 This is a cat.(改为复数形式) This is a tomato变复数形式是? this is a beautiful cat 改成复数形式 This is a knife.怎样变复数. this is a new car.变复数 This is a marker.（变复数） this is a piano怎么变复数 There is a cat on the tree.(复数) This is my book.(改为复数形式) There is a nice cat in the tree.(改为复数形式)Those are rulers.(改为单数形式) This is my book.改为复数形式 There is a nice cat in the tree.改为复数形式 Those are rulers 单数形式 This is pen变复数 There is a cat under the tree.变否定句 this is knife in the box（变复数句 This radio is on the desk.变复数 This baby is sleeping on the train变复数句 1.This is a piece of writing paper.改为复数 2.The cat is eating biscuits under the chair改为否定句