Sheffield Corp. sells two types of computer hard drives. The sales mix is 30% (Q-Drive) and 70% (Q-Drive Plus). Q-Drive has variable costs per unit of $150 and a selling price of $210. Q-Drive Plus has variable costs per unit of $180 and a selling price of $270. The weighted-average unit contribution margin for Sheffield is :A. $69. B. $210. C. $105. D. $81.