Inside an aeroplane, flying at a high altitude, A. the pressure is the same as that outside. B. normal atmospheric pressure is maintained by the use of air pumps. C. the pressure inside is less than the pressure outside. D. normal humidity and partial vacuum are maintained.

The correct answer is: C. the pressure inside is less than the pressure outside.

At high altitudes, the air pressure is much lower than at sea level. This is because there is less air above to exert pressure on the air below. To compensate for this, airplanes have pressurized cabins that maintain a higher pressure than the outside air. However, the pressure in the cabin is still lower than at sea level, which is why passengers may experience ear pain and other symptoms of altitude sickness during a flight.

Option A is incorrect because the pressure inside an airplane is not the same as the pressure outside. Option B is incorrect because normal atmospheric pressure is not maintained by the use of air pumps. Option D is incorrect because normal humidity and partial vacuum are not maintained in an airplane cabin.