一篇英语阅读，英语高手能否给我答案？

The first United States airmail stamp has an interesting story. Printed in 1918, this 24-cent stamp with a blue plane inside rose border became the center of much attention. One hundred of the stamps sold to the public became known as "inverts", for the plane was printed upside down. Singles of these "upside down" airmail stamps are now worth over $ 100,000.
The story of these stamps began on May 14, 1918, the day after they were placed on sale. In Washington D. C., W. T. Robey, a man interested in stamps, decided to buy a sheet of the new stamps and so went to the New York Avenue branch post office in Washington. When the clerk handed him a sheet of the stamps, Robey noted that they were poorly centered. He looked at other sheets and found that none was well centered. The clerk asked Robey to return later in the day when more stamps were expected.
About noon Robey came back, and the same clerk was on duty. He reached for the new sheets and handed one to Robey. The collector's heart stood still as he saw that the sheet which had been offered him had inverted centers.

题目：
6. This article is about ____.
[A] the first-class stamps eighty years ago
[B] revenue stamps in the United States
[C] the first United States airmail stamps
[D] special delivery stamps

7. These stamps were valuable because ____.
[A] the centers of them were "upside down"
[B] they were the wrong color
[C] they were off center
[D] there were so few of them

8. When Robey told his friends of the find, they ____.
[A] tried to buy his stamps
[B] also went to look for such stamps
[C] reported Robey to the government
[D] none of the above

9. The dealers knew that if there were other sheets like Robey's then ____.
[A] Robey must be lying
[B] there were no perfect ones
[C] the stamps were no good
[D] Robey's stamps would be worthless

10. Robey's stamps have seen by ____.
[A] many people
[B] no one
[C] few people
[D] everyone

CADD

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1.c 2.a 3.d 4.d