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  3. Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system whose te

Astronomers have discovered what may be five planets orbiting Tau Ceti, the closest single star beyond our solar system whose te

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问题     Astronomers have discovered what may be five planets
orbiting Tau Ceti, the closest single star beyond our solar system
whose temperature and luminosity nearly matches the sun’s. If the 【M1】______
planets are there, one of them is about the right distance from the
star to support mild temperatures, oceans of liquid water, and even life.
    Tau Ceti is only 12 light-years from Earth, just three times as
far as our sun’s nearest stellar neighbor, Alpha Centauri. Tau Ceti
assembles the sun so much that astronomer Frank Drake, who has 【M2】______
long sought radio signals from possible extraterrestrial civilizations,
made it his first target back in 1960. Unlike most stars, that are 【M3】______
faint, cool, and small, Tau Ceti is a bright G-type yellow
main-sequence star like the sun, a trait that only one in 25 stars
boasts of. Moreover, unlike Alpha Centauri, which also harbors a 【M4】______
G-type star and even a planet, Tau Ceti is single, therefore there’s 【M5】______
no second star in the system whose gravity could yank planets
away. Astronomer Mikko Tuomi of the University of Hertfordshire
in the United Kingdom and his colleagues analyzed more than
6,000 observations of Tau Ceti from telescopes in Chile, Australia,
and Hawaii. As the researchers will report in Astronomy &
Astrophysics, slight changes in Tau Ceti’s motion through space
suggest that the star may be responding to gravitational tugs from
five planets that are only about two to seven times as massive as
Earth. If that’s right, all five planets lie close to their star than Mars 【M6】______
does ours; however, Tau Ceti emits only 45% as much light as the 【M7】______
sun, so each planet receives less warmth than a planet would at the
same distance from our sun. Tau Ceti’s three innermost
planets — designated b, c, and d — are probably too hot to support
life, being such close to the star that they require only 14, 35, and 【M8】______
94 days to complete an orbit. The farthest of the three, d, is about
as close to Tau Ceti as Mercury is to the sun. It’s fourth 【M9】______
planet — planet e — that the scientists suggest might be another
life-bearing world, even though it’s about four times as massive as
Earth. If you live there, you’d see a yellow sun in the sky, but your 【M10】______
year would last just 168 days.
【M5】
选项
答案therefore—so
解析 连接词错误。这里therefore前后都是完整的句子，因此需要并列连词或复合连词连接，而therefore是副词，因此改为并列连词so。
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