13) In a class of 20 people, 15 play soccer, 14 play basketball, 13 play volleyball. How many, at minimum, practice all three sports?
(A) 0
$\begin{array}{lll}\text { (B) } 2 & \text { (C) } 7 & \text { (D) } 9\end{array}$
(E) 13.
13) The answer is (B).

Indeed, there are 5 people who do not play football, 6 who do not play basketball, and 7 who do not play volleyball: if they are all different, then exactly two people play all three sports. Obviously, it is possible that someone does not play either football or basketball, etc.; in such a case, the number of people who play all three sports increases.