$$\text { TABLE 10-5 }$$ To Test the Effectiveness of a Business School Preparation Course

$$\text { TABLE 10-5 }$$ To test the effectiveness of a business school preparation course, 8 students took a general business test before and after the course. The results are given below.
$$\begin{array} { c c c } \hline \text { Student } & \begin{array} { c } \text { Exam Score } \\\text { Before Course (1) }\end{array} & \begin{array} { c } \text { Exam Score } \\\text { After Course (2) }\end{array} \\\hline 1 & 530 & 670 \\2 & 690 & 770 \\3 & 910 & 1,000 \\4 & 700 & 710 \\5 & 450 & 550 \\6 & 820 & 870 \\7 & 820 & 770 \\8 & 630 & 610\end{array}$$
-Referring to Table 10-5, the value of the sample mean difference is ______if the difference scores reflect the results of the exam after the course minus the results of the exam before the course.

A) 50
B) 400
C) 68
D) 0