Assume that all you have available are data for the following ratios and your firm and your industry.

a. Total Asset Turnover = Revenues/Assets

b. Total Margin = EBIT/Revenues

c. Net Profit Margin = Earnings/Revenues

d. Times Interest Earned = EBIT/Interest

e. Return on Equity = Earnings/Equity

If you want evaluate the effectiveness of your company's operating managers and you can only use one of the measures, which one should it be and why?