Which of the Following Is Not One of the Reasons

Multiple Choice

Which of the following is not one of the reasons an optimization model can become nonlinear

A)  Nonconstant returns to scale. 
B)  The model objective is to minimize the sum of squared differences. 
C)  The model objective is a function of a variable times multiplied by a function of that same variable 
D)  The model objective is to minimize the sum of absolute errors.