While teaching a unit on fractions, a fifth-grade teacher does a pre-test at the beginning of the unit and a post test and the end of the unit. She found the difference of each student's scores (post-test - pre-test) and then finds the mean. There are 21 students in her class. The mean of the differences is 7 points, with a standard deviation of 3 points. The teacher wants to know if the post-test scores are different than the pre-test scores with a significance of 0.05. What is the null and alternative hypothesis? Select one: O a. Ho: /diff = 0; HA: Adiff * 0 O b. Ho: /diff = 0; HA: Adiff > 0 O c. Ho: Adiff = 0; HA: Adiff