Which of the Following Reasons Best Explains How Turbochargers Maintain

Which of the following reasons best explains how turbochargers maintain intake boost pressure (normalization) at high altitudes?

A) Greater exhaust heat from lean air-fuel mixtures at high altitudes.
B) Less resistance to compressor wheel rotation offered by lower-density air at high altitudes.
C) Greater pressure differential across the turbine wheel at high altitudes.
D) The use of a boost pressure sensor to provide closed-loop feedback control over intake pressure.