Fortune Brands Home & Security, Inc., sells Master Lock padlocks. It reported an increase in net sales from $4.0 billion in 2014 to $4.6 billion in 2015, and an increase in gross profit from $1.4 billion in 2014 to $1.6 billion in 2015. Based on these numbers, determine whether the change in gross profit represents (a) a decrease in gross profit per sale, (b) an increase in sales volume, or a combination of (a) and (b).