The correct answer is C
Explanation
Let 
stand for the number of students who belong to both clubs. The 15 members of the math club can be broken down into two groups: those who are in both clubs (there are students in this category) and those who are in the math club only (there are 
students in this category).
The 12 members of the chess club can also be broken down into two groups:students who are in both clubs and 
students who are in the chess club only.
Since a total of students belong to only one of the two clubs, you know that 
. Solving this equation gives 
, so 7 students belong to both clubs.
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