6. A deck of cards has red and black cards. If you draw one card, the probability of drawing a red card is $\frac{1}{3}$. If four more black cards are added, the probability of drawing a red card becomes $\frac{1}{4}$. Then initially, there are ( ) cards.
(A) 6
(B) 9
(C) 12
(D) 15
(E) 18
6. C.

Let the initial number of red cards be $x$. Then the number of black cards is $2 x$. Therefore, $3 x=2 x+4 \Rightarrow x=4$.
Thus, there were initially 12 cards.