4. Both Luca and Claudia have a red card and a black card in their hands. Luca draws a card at random from Claudia's hand and adds it to his own. At this point, Claudia draws a card from Luca's hand. What is the probability that each of the two ends up with two cards of the same color (one with two red cards, one with two black cards)?
(A) $3 / 4$
(B) $1 / 4$
(C) $1 / 3$
(D) $1 / 2$
(E) $1 / 6$
(4) The correct answer is $(C)$.

After Luca has drawn a card from Claudia's, Claudia is left with one card of a color, and Luca has three, of which only one is the same color as the card Claudia has left: the probability that Claudia draws that one (and they both end up with two cards of the same color) is therefore $1 / 3$.

Question proposed by Sandro Campigotto.