"My own feelings went from disbelief to excitement to downright fear", says Carl Hergenrother, 23, an Arizona undergraduate who verified a large asteroid barreling toward Earth with a 230cm telescope atop nearby Kitt Peak. "It was scary, because there was the possibility that we were confirming the demise of some city somewhere, or some state or small country". Well, not quite. Early last week, his celestial interloper whizzed by Earth, missing the planet by 450,620 km—a hairbreadth in astronomical terms. Perhaps half a kilometer across, it was the largest object ever observed to pass that close to Earth. Duncan Steel, an Australian astronomer, has calculated that if the asteroid had struck Earth, it would have hit at some 93460 km/h. The resulting explosion, scientists estimate, would have been in the 3000-to-12000-megaton range. That says astronomer Eugene Shoemaker, a pioneer asteroid and comet hunter, "is like taking all of the U.S. and Soviet nuclear weapons, putting them in one pile and blowing them all up". And what if one of them is found to be on a collision course with Earth Scientists at the national laboratories at Livermore, California, and Los Alamos, New Mexico, have devised a number of ingenious plans that, given enough warning time, could protect Earth from a threatening NEO. Their defensive weapons of choice include long distance missiles with conventional or, more likely, nuclear warheads that could be used either to nudge an asteroid into a safe orbit or blast it to smithereens. Many people including some astronomers—are understandably nervous about putting a standby squadron of nuclear tipped missiles in place. Hence the latest strategy, which in some cases would obviate the need for a nuclear defense: propelling a fusillade of cannonball-size steel spheres at an approaching asteroid. In a high-velocity encounter with a speeding NEO, explains Gregory Canavan, a senior scientist at Los Alamos, "the kinetic energy of the balls, would change into heat energy and blow the thing apart". Some astronomers oppose any immediate defensive preparations, citing the high costs and low odds of a large object"s striking Earth in the coming decades. But at the very least, Shoemaker contends, NEO detection should be accelerated. "There"s this thing called the "giggle factor" in Congress", he says. "people in Congress and also at the top level in NASA still don"t take it seriously. But we should move ahead. It"s a matter of prudence". The world, however, still seems largely unconcerned with the danger posed by large bodies hurtling in from space, despite the spectacle two years ago of Comet Shoemaker-Levy 9 riddling the planet Jupiter with mammoth explosions. It remains to be seen whether last week"s record near-miss has changed any minds. According to the text, "nuclear tipped missiles" and "cannonball-size steel spheres" (Para. 5)

A. are different in nature.
B. serve similar functions.
C. are identical to each other.
D. pose real challenges.