完形填空。 The population problem may be the 1 , one of the world today.  The world'spopulation 2 growing 3 .  Two   34  years ago, there were only 250 million people 5 the earth. Four hundred years ago, 6 number was 7 500 million.  But at the beginning of the 8 century, the world's population was about 1,700 million.  In 1970, this number was 3,600 million.  In 1990, the number was five billion.  A 9 report says 10 the world population will 11 six billion by the end of the 20th century.  This is just ten 12 after it 13 five billion.  People say that 14 the year 2011, 15 may be seven billion. (     )1. A. great (     )2. A. is (     )3. A. faster and faster (     )4. A. thousand (     )5. A. in (     )6. A.a (     )7. A. nearly (     )8. A. twenty (     )9. A. USA (     )10. A. what (     )11. A. pass (     )12. A. weeks (     )13. A. get (     )14. A. .by (     )15. A. this B.  greater B. are B.  fast and fast B.  thousands B.on B. an B.almost B.  twelve B. UN B.if B.passes B.months B.gets B.until B. it C.  greatest C. be C.  fastest and fastes C.  thousand of C.   at C. the C.over C.  twentieth C. PRC C.that C.passed C.years C.reached C.when C. one