力的大小、____ 和作用点叫力的三要素．

我国西北一些地区，如内蒙古，过去是“天苍苍，野茫茫，风吹草低见牛羊”，现在则是“天苍苍，野茫茫，老鼠跑过露脊梁”，其主要原因是(    )

根据短文内容, 选择正确答案。

    Hi, I am Andy. I am in Class 3, Grade 7. Welcome to our classroom. Isn't it big and clean?

   We all have lockers (柜) in the classroom. The red lockers are girls'. And boys' lockers are yellow. Do you like the colours? I have a locker too. This one is mine (我的). I put many things in it. There are some comic books, a football, and a pair of shoes in it. Jack's locker is beside（在……旁边） mine. There are socks and some paper in it. That one is Tom's. There are many CDs and books in it. His glasses are in the locker too.

    What is in Mary's locker? There is a yellow bag in it. What's in the bag?  We don't know. I guess there are lots of snacks(零食) in it.

— The box is too heavy to carry. What's in it?

— Jack _______ it with many books.