A firm's debt to equity ratio varies at times because: a. the market allows no leeway in the debt to equity ratio before penalizing the firm with a higher cost of capital. b. a firm will want to take advantage of timing its fund raising in order to maximize costs over the long run. c. a firm will want to sell common stock when prices are low and bond when interest rates are high. d. a company will sell bonds when interest rates are low and stock prices are high