Which of the following best describes blocking, a principle of experimental design? o a. an experiment that features many groups of subjects as well as different kinds of tests, if possible b. the separation of experimental subjects into groups that are as similar as possible to control the effects of lurking variables c. the selection of experimental subjects from a large group of units or subjects and then randomly assigning members to treatment and control groups using either a random number generator or a random number table d. a feature of an experiment designed to isolate the variable under observation