概括大意與完成句子
Tunguska Event
　　1 A hundred years ago this week, a gigantic explosion ripped (撕裂) open the day y above a forest in western Siberia, leaving a scientific 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 explosion flattened 80 million trees across an area of 2,000 square kilometers. The fireball was so great that, a day later, Londoners could read their newspapers under the night sky. What caused the so-called Tunguska Event, named after the nearby Podkamennaya Tunguska River, still remains a mystery.
　　3 Experts suspect it was a rock that, after traveling in space for millions of years, was destined to crash to Earth at exactly 7:17 a.m. on June 30, 1908. This possibility worries scientists. “Imagine an unspotted asteroid (小行星) hitting a significant chunk(塊) of land ... and imagine if that area, unlike Tunguska, were populated,” the British science journal Nature commented recently.
　　4 But no fragments of the “rock” have ever been found. Finding such evidence would be important, for it would increase our knowledge about the risk posed by dangerous Near Earth Objects (NEOs), say Italian researchers Luca Gasperini, Enrico Bonatti and Giuseppe Longo. When the next Tunguska NEO approaches, scientists will have to decide whether to try to deflect (使偏轉(zhuǎn)) it or blow it up in space.
　　5 However, several rival theories for the Tunguska Event exist. Wolfgang Kundt, a professor at Germany's Bonn University, believes the Tunguska Event was caused by a massive escape of 10 million tons of methane(甲烷)-rich gas from deep within earth's crust. Some people hold that the explosion was caused by an alien spaceship crash, or a black hole in the universe.
　　23. Paragraph 2
　　24. Paragraph 3
　　25. Paragraph 4
　　26. Paragraph 5
　　A. Competing Explanations
　　B. Unknown Attacks
　　C. Mysterious Explosion
　　D. Star War
　　E. Importance of Finding Evidence
　　F. Explanation that Worries Scientists
　　27. The gigantic explosion that occurred a hundred years ago
　　28. The shock wave which followed the dazzling light
　　29. The hypothesis that the explosion was caused by a rock colliding with the Earth
　　30. Wolfgang Kundt, who has developed an alternative theory
　　A. has remained a puzzle
　　B. lacks sufficient evidence
　　C. is a university professor
　　D. was generated by the explosion
　　E. will kill many animals
　　F. are attacked by aliens
CFEAADBC