SMS tests logical thinking.

B knows that all the cards of the suit are repeated, so he knows that A doesn't know what it is; When A listens to B, it will know that these cards are a set of cards with completely repeated colors. Well, look, the cards are all repeated. There are two groups, spades and diamonds. Hearts 2 do not repeat, clubs 9 do not repeat. In other words, if A excludes hearts or clubs, he will know what card it is. Then let's take a look again. There are spades 7, spades Q and diamonds 3 overlapping with the cards of hearts and clubs. So now there are three possible cards, but B can't guess the answer from spades, so the answer can only be box 3:)