A. Taking enough fuel.

　　B. Reusing the spaceship.

　　C. Using the airplane again.

　　D. Building the space station.

　　7. From the passage we can infer ________.

　　A. its Dragon capsule has taken the first passengers to Mars

　　B. NASA pays $5,000,000 for Russia to take one astronaut into space

　　C. the Dragon capsule is a spaceship made by NASA astronauts

　　D. Space X will use a Dragon capsule for passengers to make a trip to Mars

　　8. The main idea of the passage is ________.

　　A. the ticket to Mars will cost just $500,000

　　B. Space X is working together with NASA

　　C. the Dragon capsule is a cheap spaceship

　　D. the travel to Mars is becoming popular

　　C

　　240 BC: Greek astronomer, geographer, mathematician and librarian Eratosthenes worked out the Earth's circumference (the distance around it). His data was rough, but he wasn't far off.

　　Eratosthenes was an all­around man. Some people then called him Pentathalos, a champion of many skills. His rich knowledge made him a natural for the post of librarian of the library of Alexandria, Egypt, the greatest repository of classical knowledge.

　　Eratosthenes knew that at noon on the day of the summer solstice, the sun was observed to be directly overhead at Syene (modern­day Aswan): You could see it from the bottom of a deep well, and a sundial (日晷) left no shadow. Yet, to the north at Alexandria, a sundial left a shadow even at the solstice midday, because the sun was not directly overhead there. Therefore, the Earth must be round - already firmly believed by the astronomers of his day.

　　What's more, if one supposed the sun to be far away enough to be sending parallel (平行的) rays at Syene and Alexandria, it would be possible to figure out the Earth's circumference. Of course, his measurements were slightly off. And we don't know today the exact size of the measurement unit Eratosthenes was using when he came up with the final figure of 252,000 stades. So how big is 252,000 stades? It's somewhere between 24,663 and 27,967 miles. The accepted figure for circumference today is 24,902 miles. Pretty good for a guy without modern measurement tools.

Eratosthenes went further and worked out the extension of the year as 3651/4 days. He suggested that calendars should have a leap day every fourth year, an idea taken up two centuries