[单选题]选择正确的读音____。
A.瓜熟蒂tí落
B.瓜熟蒂dì落

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 automatically monitor life support systems, snapping photos, and taking overmonitoring tasks from failed sensors.
B. They are capable of videoconferencing so that they offer a virtual presence of ground crows and scientists.
C. They can go where no man has gone since the vehicles were first assembled.
D. All of the above mentioned.