14・18 There are two cards, one with both sides red, and the other with one side red and the other side blue. The probability of selecting either card is the same (both $\frac{1}{2}$). If a card is chosen and placed on the table, and the side facing up is red, what is the probability that the side facing down is also red?
(A) $\frac{1}{4}$.
(B) $\frac{1}{3}$.
(C) $\frac{1}{2}$.
(D) $\frac{2}{3}$.
(E) $\frac{3}{4}$.
(24th American High School Mathematics Examination, 1973)
［Solution］Let the faces of the first card (both red) be numbered 1 and 2, and the red and blue faces of the second card be numbered 3 and 4. This way, any one of the four faces $1,2,3,4$ has a chance of being drawn.

However, under the given conditions, face 4 cannot appear again, so only faces 1, 2, or 3 are possible.

Thus, among these three faces, two are red and one is blue, making the probability of drawing a red face $\frac{2}{3}$.
Therefore, the answer is $(D)$.