E
Could we "terraform" Mars —that is, change its frozen, thin-aired surface into something more friendly and Earthlike? Should we? The first question has a clear answer: Yes, we probably could．Spacecraft, including the ones now exploring Mars, have found evidence that it was warm in its youth, with rivers flowing into vast seas．And right here on Earth, we've learned how to warm a planet: just add greenhouse gases to its atmosphere．Much of the CO₂ that once warmed Mars is probably still there, in frozen dirt and polar ice caps, and so is the water．
Most of the work in terraforming, says NASA scientist Chris Mackay, would be done by life itself． "You don't build Mars," Mackay says．"You just warm it up and throw some seeds．" Powerful greenhouse gases could be produced from elements in dirt and air on Mars and blown into the atmosphere; by warming the planet, they would release the frozen CO₂, which would quicken the warming and increase atmospheric pressure to the point where liquid water could flow．Meanwhile, says botanist James Graham of the University of Wisconsin, human settlers could seed the red rock with a succession （系列） of ecosystems ?first bacteria and lichens （地衣）, which survive in Antarctica, later mosses （苔藓）, and after one thousand years or so, redwoods．Getting breathable oxygen levels out of those forests, though, could take thousands of years．
However, Mars is in no immediate danger．Some space scientists recently recommended going to the moon or an asteroid （小行星） first, and pointed out the space agency lacks the funds to go anywhere．It didn't estimate the cost of gardening a dead planet．
72．What is the most essential to make Mars fit for living?
A．Turn ice into flowing water．          B．Clean the dirty atmosphere．
C．Make the atmosphere more suitable．    D．Go to the moon or an asteroid first．
73．What does the author think of the idea ofterraforming Mars?
A．We could do it．                            B．We couldn't do it．
C．We should do it．                           D．We shouldn't do it．
74．In the botanist's opinion, man needs to __________.
A．enable Mars to clean its atmosphere
B．build a greenhouse on Mars
C．send some people to settle on Mars
D．cultivate ecosystems from lower to higher life form
75．What is the best title for the passage?
A．Making Mars the New Earth．           B．A Good Way to Change the Universe
C．Humans' Great Power．                     D．Terrible Science and Technology．