Answer A is the Bristol answer. I too got caught out with this when doing the feedback. As Mach is based on temp then I answered B. It is wrong becuase a machmeter gets Mach thus: M = (Pitot - Static) / Static. So the higher the altitude (the lower the pressure) lower the speed of sound. The machmeter automatically compensates for temprature but does not measure it. Well that is my understanding of it. If I am wrong I would like to know...