Experts have long supposed that the Red Planet was once home to water some two billion years ago, but now a team of Canadian and US researchers say they've found hard evidence to support the theory-a discovery certain to heat up discussion on whether Mars once supported life．
The researchers' study says the“ragged, kilometer-high and undulating(起伏的)”features on the Martian surface are in fact shorelines of massive ancient oceans that once covered a third of the planet's surface．
“It shows that there were shorelines and…that Mars, at one time, had massive standing bodies of water that we would call oceans．It was a blue planet．It was a blue planet early in its history, and these were not small pools,”said Jerry Mitrovica, a University of Toronto geophysicist involved in the study．
Because the study did not clearly involve biological issues, neither scientist wished to speculate(推测)on the implications(暗示)their findings may have regarding the existence of life-ancient or current-on Mars．But the study's lead author, Taylor Perron of Harvard University acknowledged that“liquid water does seem to be a basic requirement for life as we know it on Earth．”
Perron said the study further shows it's mathematically impossible for the entire Martian water supply to have been lost to space, and much of it may still be underground．So the next step in terms of research will be to figure out where the water has gone．By pointing out where the bodies of water might have been located, it will give scientists a good“target”in terms of where to concentrate exploration efforts．
(1)
The underlined word“hard”in the first paragraph is closest in meaning to ________．
[ ]
A．
true
B．
firm
C．
difficult
D．
unkind
(2)
According to the passage, the shorelines on Mars ________
[ ]
A．
now are still lying there with water
B．
will bring hot discussion about life on Mars
C．
will remain to be proved by scientists
D．
look ragged and extend kilometers long
(3)
It can be inferred from the last paragraph that ________．
[ ]
A．
man may find underground water on Mars
B．
water on Mars was completely lost into space
C．
scientists have located the position of water on Mars
D．
scientists' main target is to analyze water on Mars
