Return on investment $($ROI$)$ is commonly expressed in each of the following ways, except

ROI $=$ Net income $/(($Beginning of year total assets $+$ end of year total assets$)/2)$

ROI$=$ Operating income $\frac{?}{?}$ Average operating assets

ROI$=$ Net income $\frac{?}{?}$ Average total assets

ROI $=$ Net income $\frac{?}{?}$ Average stockholders' equity