Assume the competing hypotheses take the following form: Ho: H1 - U2 =0, HA: H1 - 12 #0, where , is the population mean for population 1 and 2 is the population mean for population 2. Also assume that the populations are normally distributed and we use independent sampling. The population variances are not known and assumed to be unequal. Which of the following expressions is the appropriate test statistic? O tof= Spin O tof 2 51 O 2 71 712