C
An oasis (绿洲) is an island of life in an ocean of temperature extremes．Any oasis always contains one or more springs Oases make it possible to survive through the desert．In large deserts such as the Sahara．towns are located around sources of water such as oases and rivers．
What causes an oasis? An oasis is actually a spot in the desert where the altitude is low enough that the water table is fight below the surface，resulting in the presence of springs．Even in a desert, it rains occasionally，and this produces a water table just above the bedrock，usually several hundred feet below the surface．Sand is very porous (可渗透的)，so most water runs right
through it and down to the bedrock．
Deserts consist of many millions of tons of sand．There is only one natural force capable of moving it in noticeable amounts：the wind．Although，in an average dust storm，ten cubic feet of air only holds about an ounce of sand，a cubic mile of air can move about 4,600 tons of it，leading to erosion (侵蚀)．A severe storm is capable of moving as much as 100 million tons of sand and dust．
In certain areas where large quantities of sand are moved by storms，erosion digs all the way down to the water table，putting it just beneath the surface Seeds planted in the ground there are capable of extending roots into the moist land，producing an oasis．
Sometimes．the oasis produced by the wind can be very large when vast tracts of desert are wiped clean by storms．The great Kharga oasis in the Sahara，for example，is over 100 miles long and 12 to 50 miles in width. The oasis was produced when erosion caused the edges of the hollow places to sink down to the water table.    
63．What does “it” in Paragraph 4 refer to_______．
A．erosion        B．water table          C．sand                      D．storm
64．How many conditions of the oasis formation are mentioned in the passage without considering
The wind and seeds?
A．3                B．4                        C．5                           D．6
65．We can infer from the passage ________．
A．there is at least one spring on an oasis
B．the bedrock is usually just right below the surface
C．the great Kharga oasis is the biggest oasis in the world
D．the great Kharga oasis is at a low altitude
66．In the last paragraph，the author mentions the great Kharga oasis as an example to_______．
A．introduce the great oasis to the readers
B．tell how the oasis was produced by rain
C．show the oasis produced by the wind can be very large
D．tell it’s easy to form oases in the desert after storms