A statistics instructor overhears students talking about paying Smarty Pants Tutoring to help them improve their ACT scores. She tells them that students almost always get a better score the second time, so she isn't sure that paying for study help is worthwhile. She samples 20 students who've used Smarty Pants to compare their second scores after help to their first scores. She then compares their improvement of that to a second group of 20 students who took the ACT twice but without any paid study assistance in between.

Select one:

a.)Two-sample T-Interval for means

b.)Two-sample Z-Interval for proportions

c.)No inference required

d.)T-Interval for the difference in matched pairs (paired data)