define: blocking a) choosing the individuals easiest to reach. b) the explanatory variable(s) in an experiment. c) using extraneous factors to create similar groups. d) directly holding extraneous factors constant. e) successively smaller groups are selected within the population in stages. f) population is divided into similar groups and a srs is chosen from each group. g) gives each member of the population a known chance to be selected. h) people who choose themselves for a sample by responding to a general appeal. i) every possible sample of a given size has the same chance to be selected.