(Requires Calculus)In the Multiple Regression Model You Estimate the Effect

(Requires Calculus) In the multiple regression model you estimate the effect on Yi of a unit change in one of the Xi while holding all other regressors constant.This

A) makes little sense,because in the real world all other variables change.
B) corresponds to the economic principle of mutatis mutandis.
C) leaves the formula for the coefficient in the single explanatory variable case unaffected.
D) corresponds to taking a partial derivative in mathematics.