14. In the annual football league of a certain middle school, it is stipulated that each team must play one match against every other team. If in the 2012 season all teams played a total of 21 matches. Then the number of participating teams is ( ).
(A) 6
(B) 7
(C) 8
(D) 9
(E) 10
14. B.

There are $n$ teams participating in the league.
$$
\text { Then } \frac{n(n-1)}{2}=21 \Rightarrow n=7 \text {. }
$$