【单选题】

For centuries, explorers have risked their lives venturing into the unknown for reasons of economic benefit and national glory. Following the lunar missions of the early 1970s, Mars now looms as humanity’s next great, unknown lanD、But with dubious prospects for short-term financial return and with international competition in space a receding memory, it is clear that imperatives other than profit or national pride will have to compel human beings to leave their tracks on the planet’s red surface.

With Mars the scientific benefits are perhaps higher than they have ever been. The issue of whether life ever existed on the planet, and whether it persists to this day, has been highlighted by accumulating evidence that Mars once had abundant liquid water and by the controversy over suggestions that fossils of bacteria rode toEarth on a rock ejected from Mars during its early history.A、definite answer about life on Mars, past or present, would give researchers invaluable data about the range of conditions under which a planet can generate the complex chemistry that leads to life. The revelation that life arose independently on Mars and onEarth would provide the first concrete clue in one of the deepest mysteries in all of science: how prevalent is life in our galaxy
One of the reasons why the idea of sending people to Mars strikes a chord in so many people is that it is already possible—the U.S. has the money and the fundamental technology needed to do it. More important, recent discoveries about the planet’s environment in the distant past have presented a clear and compelling scientific incentive for sending people: to search for evidence of life. The thesis that liquid water was once stable on Mars has been strengthened by aerial photographs taken last year that showed what appeared to be a drainage channel cut deeply by water flowing for hundreds if not thousands of years.
A、thorough hunt for any life on Mars that might be hanging on—despite the present deficit of water—would also have to be undertaken by humans, according to some experts. Such life will be hidden and probably tiny. "Finding it will require surveying vast tracts of territory," one expert explains. "It will require the ability to cover long distances and adapt to different conditions." Robots might be up to the task sometime in the distant future, making human explorers redundant, he concedes.But relying on them to survey Mars during periodical missions to the planet would take a very long time— "decades if not centuries," he believes.
The aerial photographs taken last year indicate that ______.A．there did exist liquid water on Mars
B．a drainage channel was cut deeply by Martians
C．there isn’t any life on Mars
D．water had flew on Mars for hundreds to thousands of years