解析：此处表示我帮助这个老太太的时候。

　　答案：D

　　20．A.purer

　　B．more difficult

　　C．happier D．more significant

　　解析： 我帮助了他人，这一时刻比我几个星期里的任何时候都更让我感到快乐。

　　答案：C

　　.阅读理解

　　(2017·东北三校第二次联考)One of the world's great natural wonders is in New Mexico, in the southwest of America. Nature has created huge moving hills of pure white sand. These sand dunes cover more than seventy­thousand hectares of desert.

　　It is one of the largest sand dune fields in the United States. The bright white sand dunes are always changing，always moving, like waves on the ocean.Driven by strong winds, the sand moves and covers everything in its path. It is like a huge sea of sand.

　　You may wonder how anything can live in this extreme environment of a white sand desert. There is not much rain.The heat in summer is intense.

　　The sand lacks nutrients.Plants and animals struggle to survive. A few kinds of plants grow quickly to survive burial by the moving sand dunes. Several kinds of small animals have become white in color in order to hide in the sand.

　　More than five hundred thousand people visit White Sands each year. They climb on the dunes and observe the moving sea of sand.

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