Amarketing research firm wishes to determine the most effective method of selling kitchen utensils: print, radio, television, or online. the researcher segments the volunteers by their ages. of the 420 volunteers, 90 are under 20 years old, 70 are 20-39 years old, 130 are 40-59 years old and 130 are 60 years old or older. the volunteers from each group are randomly assigned to either the print advertising group, the radio group, the television group, or the online group. each group is exposed to the advertising. after 3 hours, a recall exam is given with the proportion of correct answers recorded what type of experimental design is this? o a. matched pair o b. randomized block design o c. survey o d. completely randomized assignment what is the response variable in this experiment? o a. the scores on the recall exam o b. the scores on the pretest ° c. the ages of the subjects 0 d. the type of advertising what explanatory variable is manipulated? 0 a. the time before recall exam o b. the type of advertising