[单选题]列车最后（ ）不得为关门车。
A.四辆
B.三辆
C.两辆
D.一辆

⁶⁰钴治疗时，骨和软组织吸收剂量（）

In about two years, NASA astronauts will go into space with personal assistants. Robotic assistants, of course. About the size of softballs, these personal satellite assistants (PSAs) can float throughout the space shuttle or space station, automatically monitoring life support systems, snapping photos, and taking over monitoring tasks from failed sensors. They’re equipped with collision avoidance system so they won’t bump into anything Naturally, they’re wireless. Thanks to their videoconferencing capabilities, the PSAs offer a virtual presence for ground crews and scientists, letting them advise astronauts--in real time --in the midst of their experiments. The PSAs. will be stowed until the spacecraft reaches orbit (天体运行的轨道). And then activated by the astronauts, ground crews, or even by the spacecraft itself or other PSAs requesting help, they can work automatically or respond to voice or radio commands, and can exchange information with the spacecraf
A. PSAs can deal with all kinds of affairs in space station or space shuttle.
B. PSAs are those who can shoulder more responsibilities than astronauts.
C. PSAs have so many benefits that astronauts can easily carry out their tasks.
D. Nothing can be done without PSAs.