Teachers Jack Black and Shannon Shack Want to Increase the Percentage

Teachers Jack Black and Shannon Shack want to increase the percentage of high school freshmen who pass an algebra exam (dependent variable) .They conduct experiments on five sets of classes (named A to E) in Toronto area schools.They randomly assign students and give a pretest in September followed by a post-test the following January.The treatment is teaching method.The experimental groups are taught with special in-class demonstrations and pictorial gamesquizzes on computers,but no homework assignments,textbooks,or tests.The control groups are taught in the traditional manner with a textbook,homework,formulas on the blackboard,and tests.
$\begin{array} { l | c c } & { \text { \% Passing the Math Exam } } \\& \text { Pretest Score } & \text { Posttest Score } \\\hline \text { A } & & \\\text { Experimental Group } & 20 \% & 81 \% \\\text { Control Group } & 19 \% & 62 \% \\\text { B } & & \\\text { Experimental Group } & 33 \% & 87 \% \\\text { Control Group } & 15 \% & 56 \% \\\text { C } & & \\\text { Experimental Group } & 22 \% & 77 \% \\\text { Control Group } & 25 \% & 76 \% \\\text { D } & & \\\text { Experimental Group } & 66 \% & 69 \% \\\text { Control Group } & 67 \% & 98 \% \\\text { E } & & \\\text { Experimental Group 1 } & 21 \% & 79 \% \\\text { Control Group 1 } & 20 \% & 66 \% \\\text { Experimental Group 2 } & & 68 \% \\\text { Control Group 2 } & & 56 \%\end{array}$
-Which set of classes appears to have had serious “selection bias” problems in assignment to the experimental and control groups?

A) A
B) B
C) C
D) D
E) E