28. The principal reason for turbocharging is to increase the power output of an engine of given size and speed, by enabling the cylinders to be charged with air at an elevated pressure and, therefore, at a higher density than air at atmospheric pressure. What is the main purpose to use a turbocharger？ (A) To elevate the density of air in order to decrease the pressure in cylinder. (B) To elevate the pressure of air in order to decrease the density in cylinder. (C) To improve the power output of an engine of given size and speed by increasing the compressed air intake temperature. (D) To improve the power output of an engine of given size and speed by increasing the density of charged air.