4. Clara wants to lend two of her books to her cousin Luca. She owns 9 mystery books, 7 travel books, 4 poetry books. In how many ways can the pair of books be chosen, considering that they must be of different genres?
(A) 40
(B) 121
(C) 118
(D) 120
(E) 127
(4) The correct answer is $(E)$.

The two books can be:

- either a mystery book and a travel book (with $9 \cdot 7=63$ possible choices);
- or a mystery book and a poetry book (with $9 \cdot 4=36$ possible choices);
- or a travel book and a poetry book (with $7 \cdot 4=28$ possible choices).

In total, the possible choices are therefore $63+36+28=127$.

Question proposed by Paolo Francini.