The HR department of a large company wants to determine how often to bring representatives from the financial firm managing employee pensions on site to meet with individuals about their retirement plans. In order to determine level of interest, they decide to survey employees. Suppose they group employees by age categories (e. g., under 30; 30 – under 45; 45 – under 60, 60 or older) and randomly select 50 individuals from each category. This sampling plan is called . a) stratified samplingb) simple random samplingc) cluster samplingd) convenience samplinge) systematic sampling