Questions 35 to 39 are based on the following passage.After men landed on the moon in 1969, astronauts (宇航员) around the world had a problem – there were no other places they could go! Even today, the other planets are still too far away for astronauts to fly to. So while rockets and robots can go to other planets, manned flights have to stay closer to home. Therefore, manned space programs have turned their attention to solving problems related to living and working in space.Currently, NASA’s (US National Aeronautics and Space Administration) manned space exploration program focuses on the space shuttle program. NASA now operates three space shuttles, Discovery, Atlantis, and Endeavor. Unfortunately, two of NASA’s shuttles, Challenger and Columbia, were lost through accidents. Seven astronauts died in each accident. The program completed 135 missions when the program ended with the successful landing at the Kennedy Space Center on July 21, 2011. These missions have included putting satellites into orbit, photographing the earth, studying space, conducting experiments related to working in space, and connecting with various manned space station in orbit.Throughout the short history of the exploration of space, several space stations have been put into orbit. The first manned space station was the Soviet station Salyut 1, put into orbit in 1971. Later, in 1986, the Soviet Union launched the Mir space station. Mir stayed in orbit until March 23, 2001. Over that time, 104 astronauts visited the station to stay for various lengths of time. The person who has spent the longest in space so far is Russian astronaut Valeri Polyakov. Working as the doctor aboard the station, he lived on Mir for 438 days without returning to earth. In total, Polyakov worked aboard Mir for 678 days before retiring.Today, astronauts from around the world are working together to complete the International Space Station (ISS). The construction began in 1998, and the US Orbital Segment was completed in 2011. Operations are expected to continue until at least 2020. In the long run, it is hoped that the ISS will be a place where people can live and work all year round.35. Why can’t astronauts travel to other planets now?

A. Because there are not enough space shuttles.
Because there have been too many rocket accidents.
C. Because the journey would take too long for human.
D. Because there are too many problems here on Earth.