Which of the following statements is incorrect? (In each one of the following options analyse your way of thinking to justify your decision). (a) The quickest way to determine whether the firm has too much debt is to calculate the debt ratio. (b) The best rule of thumb for determining the firm's liquidity is to calculate the current ratio. (c) From an investor's point of view, the rate of return on common equity is a good indicator of whether or not a firm is generating an acceptable return to the investor. (d) The operating margin is determined by expressing net income as a percentage of total sales