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次の文章を読んで、 問1~4に答えなさい。
The establishment of a colony on Mars has been a dream for decades.
Inevitably some people have objected to the idea of colonizing Mars on both
ideological and practical grounds. Some object to humans living on Mars
because they would harm whatever bacterial life forms might be present on the
planet. Others oppose Mars settlements because they disagree with the idea of
using the Red Planet as a "backup" in case the Earth is destroyed. Those in
favor of colonizing Mars, however, look to spread the human race beyond our
single planet. The practical considerations of surviving long term on a world
without a breathable atmosphere, no surface water, exposure to radiation, and
extremes of heat and cold all have to be addressed first. Mars colonists could
survive in domed cities, extracting and recycling resources from the Martian
environment. However, a more interesting plan for the settlement of the Red
Planet involves a process called terraforming, turning the dangerous environ-
ment of Mars into something resembling Earth.
Billions of years ago, Mars was more like Earth, with a thick atmosphere as
well as oceans and rivers of surface water. The planet may well have had
complex life forms. However, sometime in the distant past, Mars lost its (A
). When Mars found itself without the protection of that field, solar wind
relentlessly stripped it of its atmosphere, quickly turning the planet into the
frozen desert it currently is. While a number of schemes exist to restore Mars'
atmosphere, creating a runaway greenhouse process that would raise its
temperature, NASA and some academic researchers recently came up with a
simple way to achieve the process naturally. The idea involves the creation of
an electromagnetic shield between Mars and the Sun to protect the Red Planet
from solar wind. Without the solar wind stripping it away, the atmosphere of
Mars would gradually become thicker. Soon the temperature on the Martian
surface would become high enough to release the trapped ( B ) at the poles,
accelerating the (C). Water ice at the poles would melt, giving Mars back
some measure of its oceans and rivers. All humans would have to do is introduce
