单项选择题

Tunguska Event 　　1 A hundred years ago this week, a giganticexplosion ripped (撕裂) open the day y above a forest in western Siberia, leaving ascientific riddle that endures to this day. 　　2 A dazzling light pierced the heavens,followed by a shock wave as strong as 1,000 atomic bombs. The explosionflattened 80 million trees across an area of 2,000 square kilometers. Thefireball was so great that, a day later, Londoners could read their newspapersunder the night sky. What caused the so-called Tunguska Event, named after thenearby Podkamennaya Tunguska river, still remains a mystery. 　　3 Experts suspect it was a rock that, aftertraveling in space for millions of years, was destined to crash to Earth atexactly 7:17 a.m. on June 30, 1908. This possibility worries scientists.“Imagine an unspotted asteroid (小行星) hitting a significant chunk(块) of land ... and imagine if that area, unlike Tunguska, werepopulated,” the British science journal Nature commented recently. 　　4 But no fragments of the “rock” have everbeen found. Finding such evidence would be important, for it would increase ourknowledge about the risk posed by dangerous Near Earth Objects (NEOs), sayItalian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo. When thenext Tunguska NEO approaches, scientists will have to decide whether to try todeflect (使偏转) it or blowit up in space. 　　5 However, several rival theories for theTunguska Event exist. Wolfgang Kundt, a professor at Germany’s Bonn University,believes the Tunguska Event was caused by a massive escape of 10 million tonsof methane(甲烷)-rich gasfrom deep within earth’s crust. Some people hold that the explosion was causedby an alien spaceship crash, or a black hole in the universe. Paragraph 3