a university runs a new class designed to improve the freshman experience. the researchers want to evaluate whether participation in the new class will improve attitudes toward school. to evaluate the hypothesis, the research team uses a t test statistic to compare the sample mean from a sample to the mean of a national norm group. the population mean is 20, the mean from a sample of 25 students is 23, and the standard error is 2. what is the null hypothesis for the analysis?