Deserts are found where there is little rainfall or where rain for a whole year falls in only a few weeks' time. Ten inches of rain may be enough for many plants to survive if the rain is spread throughout the year, If it falls, within one or two months and the rest of the year is dry, those plants may die and a desert may form.   
Sand begins as tiny pieces of rock that get smaller and smaller as wind and weather wear them down. Sand dunes (沙丘) are formed as winds move the sand across the desert. Bit by bit, the dunes grow over the years, always moving with the winds and changing the shape. Most of them are only a few feet tall, but they can grow to be several hundred feet high.
There is, however, much more to a desert than sand. In the deserts of the southwestern United States, cliffs (悬崖) and deep valleys were formed from thick mud that once lay beneath a sea more than millions of years ago. Over the centuries, the water dried up. Wind, sand, rain, heat and cold all wore away at the remaining rocks. The faces of the desert mountains are always changing –-very, very slowly ---as these forces of nature continue to work on the rock.
Most deserts have a surprising variety of life. There are plants, animals and insects that :have adapted to life in the desert. During the heat of the day' a visitor may see very few signs of living things, but as the air begins to cool in the evening, the desert comes to life. As the sun begins to rise again in the sky, the desert once again becomes quiet and lonely.
68. Many plants may survive in deserts when__________________.
A. the rain is spread out in a year      B. the rain falls only in a few weeks
C. there is little rain in a year         D. it is dry all the year round
69. Sand dunes are formed when___________________.
A. sand piles up gradually             B. there is plenty of rain in a year
C. the sea has dried up over the years    D. pieces of rock get smaller
70. The underlined sentence in the third paragraph probably means that in a desert there is____________.
A. too much sand              B. more sand than before
C. nothing except sand          D. something else besides sand
71. It can be learned from the text that in a desert____________.
A. there is no rainfall throughout the year               B. life exists in rough conditions
C. all sand dunes are a few feet high              D. rocks are worn away only by wind and heat