__9__Scientists have learnt that Mars seems to be the planet with the best conditions for life after Earth.__10__The reason is that the air on Mars is mainly made up of carbon dioxide，nitrogen，and argon，all of which humans cannot breathe，and has only 0.13% oxygen，while the oxygen content of the air on Earth adds up to 21%.

　　__11__We now know there isn't enough oxygen on Mars for humans to breathe，but there are signs of water there.NASA scientists have found ice on Mars，and there is some water in the atmosphere，The Phoenix Mars Lander landed on the planet on May 25,2008，to find out if there was life there，and to explore the planet's climate and geology.__12__A physicist once said，“We agree that life can exist under different conditions.We now believe that there might have been some form of life on Mars billions of years ago，maybe deep underground.__13__On the question of humans going to live on Mars well，first of all，we have to sort out some problems.And then... who knows？”

　　A．The physicist is not sure whether people will live on Mars one day.

　　B．Whether life can exist on Mars has been a question in the minds of space experts for a long time.

　　C．Humans can't breathe on Mars because there is too little air there.

　　D．As far as we know，for life to exist，two essentials are needed—air and water.

　　E．However，if people went to live on Mars tomorrow，they wouldn't be able to breathe.

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