Apple Assembles Most IPhones in China Because the Lower Wages

Apple assembles most iPhones in China because the lower wages earned by Chinese workers and their experience in electronics manufacturing reduce the costs of assembling iPhones. This is an example of Apple reacting to which of the three key economic ideas?

A) People are rational.
B) People respond to economic incentives.
C) Optimal decisions are made at the margin.
D) Firms attempt to maximize revenues.