'D' was marked as incorrect. Need assistance with the correct answer.

Sensitivity analysis is the form of risk analysis

a	that examines the relationship between total firm cash flows and the NPV of a particular project.
b	that examines the volatility of NPV.
c	that examines the impact of key variables such as sales or costs one at a time.
d	that examines the impact of key variables such as sales or costs in various combinations.

D