A tutor promises to improve GMAT scores of students by more than 54 points after three lessons. To see if this is true, the tutor takes a sample of 49 students' test scores after and before they received tutoring. The mean difference was 57 points better after tutoring, with a standard deviation of the differences equal to 11 points. Let_Ddenote the mean of the difference: score after tutoring minus score before tutoring. Which of the following is the correct value of the test statistic?

Multiple Choice

• z= 1.8734
• z= 1.9091
• t₄₈= 1.8734
• t₄₈= 1.9091