Six Second Grade Students Tried Tossing a Ball into a Basket

Multiple Choice

Six second grade students tried tossing a ball into a basket ten times each. Their teacher then suggested a different way of tossing the ball, and the six students tried again. The number of successful tosses, before and after the teacher's added instruction, are shown below.
$\begin{array}{|c|c|c|}\hline \text { Student } & \begin{array}{c}\text { Successful Tosses } \\\text { Before (out of 10) }\end{array} & \begin{array}{c}\text { Successful Tosses } \\\text { After (out of 10) }\end{array} \\\hline \text { A } & 6 & 2 \\\hline \text { B } & 3 & 3 \\\hline \text { C } & 6 & 2 \\\hline \text { D } & 3 & 6 \\\hline \text { E } & 6 & 2 \\\hline \text { F } & 3 & 3 \\\hline\end{array}$
Ignoring the differences of 0 , the positive rank sum is:

A) +9
B) +3
C) -4
D) +1