Fortune Brands Home & Security, Inc., sells Master Lock padlocks. It reported an increase in net sales from $ 3.3 billion in 2011 to $ 3.6 billion in 2012, and an increase in gross profit from $ 1.0 billion in 2011 to $ 1.2 billion in 2012. Based on these numbers, determine whether the change in gross profit was caused by an increase in gross profit per sale, an increase in sales volume, or a combination of the two.