We wish to study the advertising expenditures for the 200 largest companies in Canada. Suppose the objective of the study is to determine whether firms with high returns on equity (a measure of profitability) spent more of each sales dollar on advertising than firms with a low return or deficit. To make sure that the sample is a fair representation of the 200 companies, the companies are grouped on percent return on equity What is this type of sampling called? Question 3 options: a) Simple random b) Stratified random c) Cluster d) Systematic