【单选题】

除非在平稳的空气中，否则不要超过这个速度或马赫数，而且要小心。

A、Do not go beyond this speed or Mach number besides in steady air, and then be careful.
B、Do not increase by this speed or Mach number unless in steady air, and be careful.
C、Do not exceed this speed or Mach Number except in smooth air and then only with caution.