29．什麼是馬赫 Mach number?
(A)音速‧The sound speed.
(B)真速比‧The ratio of true speed.
(C)真 速和當時音速比‧The ratio of true airspeed to the local speed of sound.
(D) 真速和音速比‧The ratio of true airspeed to the speed of sound.

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