单项选择题

What is the passage mainly concerned about

A．Physiological problems of space flight.
B．Speed and gravity of space flight.
C．Gravity and acceleration of space flight.
D．Acceleration and deceleration of space flight.

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单项选择题

Acceleration and deceleration are alike in all the following aspects EXCEPT______.

A．Both of them have the same properties
B．Both of them have the same physiological effects
C．In both processes, the same precautions must be taken
D．The physiological effects are negligible in both of them