问题：
[单选题]Astronauts now ____________: Pilots, the astronauts who actually fly the space shuttle, account for one-third of the astronaut corps. Mission specialists run experiments in orbit, work the shuttle's robot arm and go on spacewalks. They're doctors, engineers and scientists and can command stays on the space station. They are the remaining two-thirds of the corps.
Asplit into two groups
Bcome in two varieties
Cdivide into two kinds
