Scientists have found what look like cav

Scientists have found what look like caves on Mars(火星), and say they could be protecting life from the planet’s terrible environment.
The first caves discovered beyond the Earth appear as seven mysterious black dots on the pictures sent back by NASA’s Mars Odyssey orbiter. Each as large as a football field, they may be openings into natural caves below the Martian surface.
“If there is life on Mars, there is a good chance you’d find it in caves,” said Jut Wynne, one of the researchers who noticed the features while working on a US Geological Survey Mars Cave Detection Program.
Jonathan Clarke, a geologist with the Mars Society of Australia, yesterday described the discovery as exciting.
One photo taken at night by an infrared imager(红外线成像器) showed one hole to be unusually warm, suggesting hot air trapped during the day is flowing out.
“I said: ‘Wow, that’s a cave’” Dr. Clarke said excitedly. “People have been looking for these for a long time; now we have found them.”
He agreed such caves would be perfect places to hunt for life escaping from the bitterly cold, radiation-soaked(充满辐射的), dry surface.
“Tiny drops of water could collect inside,” he said. “If there are gases coming out, they could provide energy for a whole range of bacteria. A cave is also a protection from radiation; the surface of Mars is exposed to high levels of space radiation.”
The caves probably formed when tube-shaped lave flows(管状岩浆流) spread across the planet long ago. The outside of the tubes cooled, forming solid walls, while something hotter inside allowed the remaining have to flow out, forming caves. 
小题1:What does the passage mainly talk about?

A．How the caves were formed on Mars.	B．How scientists found these caves on Mars.
C．Caves on Mars may be full of hot air or a sign of life.
D．Scientists have completely recognized the surface of Mars.

小题2:We can learn from the passage that           .

A．water has already been found on Mars	B．the scientists found all the caves at night
C．it is certain that there is life in these caves
D．the surface of Mars is bitterly cold, radiation-soaked and dry

小题3:According to the passage, Dr. Clarke was so excited because        .

A．such caves could provide energy for life	B．they had finally found the caves on Mars
C．such caves would be perfect places to hunt for life
D．scientists had long been looking for these caves

小题4:Necessary conditions for life on Mars mentioned in the passage may include        .

A．lava and energy	B．water and radiation from space
C．gases and lava	D．water and protection from radiation