Selling price per unit$ $20$$ $15$Variable cost per unit$129$Contribution margin per unit$ $8$$ $6$Labor time$4$minutes$2$minutes

Roberto, Incorporated manufactures products A and B$.$ Both products have a contribution margin ratio of $40\%.$ What is the contribution margin per unit of the constrained resource for product B$,$ if labor time is the constrained resource?

multiple choice

$$2$ per minute

$$3$ per minute

$$6$ per minute

$$8$ per minute