Assume you are using net sales as the base in vertical analysis. Cost of goods sold in 2013 is 60%, and is 70% in 2014. This would always indicate that: O a. Gross Margin as a percentage of Net sales has declined. O b. cost of goods sold as a percentage of net sales has declined. O c. gross margin has declined, cost of goods sold as a percentage of net sales has increased, and the dollar amount of cost of goods sold has increased O d. gross margin has declined