A professor of women's studies is interested in determining if stress affects the menstrual cycle. Twelve women are randomly sampled for an experiment and randomly divided into two groups. One of the groups is subjected to high stress for two months while the other lives in a relatively stress-free environment. The professor measures the menstrual cycle (in days) of each woman during the second month. The following data are obtained. High stress 20 23 18 19 22 24 Relatively stress free 26 31 25 26 30 32 The obtained value of the appropriate statistic is tobe - -4.71 tobt - -3.05 tobt - -0.73 O toby - -4.69