D
Mars（火星）appears to be flowing with small streams of salty water，at least in the summer，scientists reported Monday．“It suggests that．it would be possible for there to be life today on Mars，”NASA's science mission chief，John Grunsfeld，said at a news conference on September 28，2015．
The streams are about 12 to 15 feet wide and 300 feet or more long，scientists said．“What we're dealing with is wet soil．thin layers of wet soil，not standing water，”said Aifred McEwen of the University of Arizona at Tueson！the principal scientist for the Mars Reconnaissance Orbiter's high-resolution imaging experiment．
Because liquid water is essential to life，the findings could have major implications for the possibility of Martian life．The researchers said further exploration is needed to determine whether microscopic life exists on the planet．
The presence of liquid water could also make life easier for astronauts visiting or living on Mars．Water could be used for drinking and for creating oxygen and rocket fuel．NASA's goal is to send humans there in the 2030s．
The evidence of flowing water consists largely of dark，narrow streaks(条痕) on the surface that tend to appear and grow during the warmest Martian months and fade the rest of the year．
Mars is extremely cold even in summer，and the streaks are in places where the temperature is as low as minus 10 degrees Fahrenheit．But salt can lower the freezing point of water and melt ice．
The source（源头）of the water is a mystery．Scientists noted it could be melting ice．It could be an underground aquifer，which is rock or sand that can hold water．It is possibly water vapor from the thin Martian atmosphere，Or it may be a combination Michael Meyer，lead scientist for NASA's Mars exploration program，said the only definitive way for now to determine whether there's life on Mars is to collect rocks and soil for analysis on Earth - something a U．S．lander  set for lift-off in 2020 will do．
What would be the best title for the text？

A．Streams of Water Spotted on Mars

B．Astronauts will Land on Mars

C．NASA's Mars Exploration Program

D．A Breakthrough in the Exploration of Mars

According to John Grunsfeld，there seems to be ________on Mars．

A．salty water

B．standing water

C．rocket fuel

D．human beings

Scientists are uncertain about the __________of the water on Mars．

A．state

B．source

C．temperature

D．taste

What matters to the existence of liquid water on Mars？

A．The distance from the sun．

B．The favorable climate．

C．The gravity of the earth

D．The salt in it．