mathematical concepts in ways that make them seem clear and obvious. (北京2012阅读理解A篇)

3. call on

【点拨】call on意为：①访问，拜访，探望；②邀请，要求（某人讲话等）；③请求，要求（某人做某事），要求......介入，号召，呼吁。如：

I shall call on Sandra tomorrow.

I call on Mr. Brown, the first speaker on my list.

They called on the workers of the country to support the strike.

【考例】They call on people and the government to take measures to fight against it. (天津2013阅读理解B篇)

4. pile

【点拨】pile既可作名词也可作动词。

★pile作名词，意为：①摞，垛，沓；②堆，成堆的东西。如：

Mr. Smith arranged the documents in neat piles.

There is a large pile of sand.

★pile作动词，意为：①堆放，摞起，叠放；②放置，装入；③蜂拥，拥挤。如：

My mother piled the boxes one on top of the other.

The girl piled everything into her suitcase.

The bus finally arrived and we all piled on.