The “canals” on Mars have proved to be according to the passage.（　　）

A．the minor geologic characteristics of Mars

B．natural valleys on the surface of Mars

C．rivers that have kept flowing since ancient times

D．a system that irrigates the whole surface

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1 What are the Mars’ seasonal changes in color believed to be ?（　　）

A．The natural changes on the plane

B．The seasonal cycles .

C．The movement of some life form .

D．The storm of dust in its atmospher

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2 How is the 1996 discovery related to the possible existence of organisms on Mars?（　　）

A．The meteorite containing organic compounds is part of Mar

B．A Martian rock struck Earth about 11,000 years ag

C．The organisms came back to life after the collision with Eart

D．The inhabiting organisms appeared more than three billion years ag

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3 How many arguments in this passage lead to the belief of the existence of life on Mars?（　　）

A．Tow

B．Three

C．Four

D．Five

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