A. Decreasing the selling price of tridol would decrease its unit contribution margin. The decrease in the unit contribution margin would increase the firm's overall breakeven point.

B. The installation of new computer-controlled machinery and the layoff of assembly-line workers would increase fixed costs. However, the unit contribution margin would increase as a result of the layoffs. Whether the breakeven point would increase or decrease would depend upon the magnitude of these two changes.

C. Petrol is the most profitable product for the firm, so an increase in the sales of petrol would increase the weighted average unit contribution margin of the three products. The increase in the weighted average unit contribution margin would decrease the breakeven point (Fixed costs/average unit contribution margin = breakeven point in units).

D. An increase in the overall market for septine would not, by itself, affect the firm's overall breakeven point because it would not change either the firm's total fixed costs or septine's unit contribution margin.