there ____ a pencil,two pens and there books on the desk,but

there ____ a pencil,two pens and there books on the desk,but There are a pencil and two pens on the desk. There ____ pencil-box,two books and some flowers on the desk There( are )(a book)and (two pens)(on) the desk There are a book and two pens on the desk . there were a book and two pens on the desk There a book and two pens on the desk. There is a book and two pens on the desk. There ________ a ruler and two pens on the desk. There 什么a pen and two books on the desk There are some water and two pens on the desk. There __ a book and two pens on the desk.答案是is.为什么