When Neil Armstrong and Buzz Aldrin returned from the moon, their cargo included nearly fi

fty pounds of rock and soil, which were packed in an aluminum box with seals designed to maintain the lunar surfaces low-pressure environment. But back at Johnson Space Center, in Houston, scientists discovered that the seals had been【C1】______—by moon dust. Lunar dust is fine, like a powder,【C2】______it cuts like glass. Its formed when shooting stars【C3】______on the moons surface, heating its rocks and dirt and reducing them to fine particles. Since theres no wind or water to smooth【C4】______edges, the tiny grains are sharp and uneven, and【C5】______nearly everything. "The intruding【C6】______of lunar dust represents a more challenging engineering design issue, as well as a【C7】______issue for settlers, than does radiation," wrote Harrison Schmitt, an Apollo 17 astronaut, in his 2006 book, "Return to the Moon." The dust damaged space-suits and ate away layers of moon boots. Over the【C8】______of six Apollo missions, not one rock box【C9】______its vacuum seal. Dust followed the astronauts back into their ships, too. According to Schmitt, it smelled like gunpowder and made breathing【C10】______. No one knows precisely what the extremely small particles do to human lungs. The dust not only【C11】______the moons surface, but floats up to sixty miles【C12】______it—as an outer part of its atmosphere, where particles【C13】______the moon by gravity, but are so thin that they【C14】______collide. In the nineteen-sixties, Surveyor probes filmed a glowing cloud floating just above the lunar surface during sunrise. Later, Apollo 17 astronaut Gene Cernan, while orbiting the moon, recorded a【C15】______phenomenon at the sharp line where lunar day meets night. Cernan【C16】______a series of pictures illustrating the changing【C17】______; streams of particles popped【C18】______the ground and hovered, and the resulting cloud came into sharper focus as the astronauts orbiter approached daylight. Since theres no wind to form. and【C19】______the clouds, their origin is something of a mystery. Its【C20】______that theyre made of dust, but no one fully understands how or why they do their thing.

【C1】

A．destroyed

B．stained

C．changed

D．consolidated