Scientists have found what look like caves on Mars , 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  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 , 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  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. According to the passage, Dr. Clarke was so excited because_.
such caves could provide energy for life they had finally found the caves on Mars such caves would be perfect places to hunt for life scientists had long been looking for these caves
Question: Scientists have found what look like caves on Mars , 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  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 , 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  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. 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
Answer: