阅读下面短文，从每题所给的A、B、C、D四个选项中选出最佳选项。
Do you want to become an astronaut? If you want to be one of the few people to experience the thrill of lift-off(发射), see the Earth from on high and float(漂浮) in a spacecraft, then how do you go about it?
First, you must really want to become an astronaut as it takes many years of study and work before you even begin your astronaut training. Most astronauts begin when they are between 27 and 37. Second, you need to be clever enough to attend a university to study engineering, medicine or one of the sciences. Many astronauts also learn to be pilots in their country's air force(空军). Third, astronauts come from many different countries, so they need to speak English. Then they can all talk to one another. Fourth, you must be healthy as astronaut training can be very tiring. Lastly, on a spacecraft, astronauts live and work in a very small space, so you need to get on well with people.
Are you still interested? If so, maybe you will become one of the men or women that orbit the Earth in a spacecraft, walk in space or visit Mars. See you in space!
，则下面四个数中最小的是
