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In designing an experiment involving a treatment applied to 4 test subjects, researchers plan to use a simple random sample of 4 subjects selected from a pool of 31 available subjects. (Recall that with a simple random sample, all samples of the same size have the same chance of being selected.) Answer the questions below. a.) How many different simple random samples are possible? b.) What is the probability of each simple random sample in this case? (Type an integer or a simplified fraction.)