One of the Requirements for Conducting Inference on the Least-Squares

One of the requirements for conducting inference on the least-squares regression model is that the

A) mean of the response variable changes at a constant rate while the standard deviation remains constant.
B) mean of the explanatory variable changes at a constant rate while the standard deviation remains constant.
C) mean of the explanatory variable remains constant while the standard deviation changes at a constant rate.
D) mean of the response variable remains constant while the standard deviation changes at a constant rate.