Progolf Corp. sells a variety of products, including golf balls and golf shoes. Assume that, like many other companies, Progolf reported a drop in net sales from $8.6 million in 2013 to $7.6 million in 2014, and a drop in gross profit from $4.0 million in 2013 to $3.6 million in 2014. Based on these numbers, determine whether the drop in gross profit was caused by a decline in gross profit per sale, a decline in sales volume, or a combination of the two.