a box of pencils是单数还是复数

　　I found a box of pencils.

　　我找到了一盒铅笔。

　　It is for that purpose, again, that I have bought a box of paints and some pencils.

　　正因为如此，我又买了一盒颜料和几支彩笔。

　　Some schools lack money for basics like books and pencils.

　　有些学校缺少资金购买书本、铅笔之类的基本用具。

　　They pause and chew their pencils

　　他们停了下来，咬着铅笔。

　　Those pencils are mine.

　　那些铅笔是我的。