Answer: B

Explanation:

The correct answer is B (And variable cost are constant). The formula for total contribution is given as:

$\text{Total Contribution }=\text{Units Sold }\times \text{Contribution per Unit}$

This formula is valid only if the contribution per unit remains constant, which means that both the selling price and the variable cost per unit must remain unchanged. If either the selling price or the variable cost varies, the contribution per unit would change, making the formula invalid.