9. Carlo has forgotten the unlock code for his phone. All he remembers is that the code is made up of 4 digits and the product of these digits is 18. How many possible codes meet these conditions?
(A) 32
(B) 36
(C) 40
(D) 60
(E) 24
Problem 9. The answer is (B).

Since $18=2 \cdot 3^{2}$, the digits that can appear in the unlock code are $1, 2, 3, 6$, and 9.

Codes that contain the digit 9 must also contain the digit 2 and the digit 1 repeated twice: the number of such codes is equal to $4 \cdot 3=12$: once the position of the 9 is chosen (in 4 possible ways), the position of the 2 can be chosen in 3 possible ways, after which the 1s are forced into the remaining positions: thus, 12 possibilities in total.

Codes that contain the digit 6 must also contain the digit 3 and the digit 1 repeated twice, and the number of such codes is equal to the previous one (12). Codes that contain the digit 3, different from those considered so far (i.e., without 6 or 9), must necessarily contain it twice, together with a 2 and a 1; the number of such codes is therefore still equal to 12 (once the positions of 1 and 2 are chosen, the other two positions are forced to each host a 3). Finally, any code that contains the digit 2 must necessarily contain a 9 or two 3s, and such codes have already been considered.

In total, therefore, the number of possible codes is $3 \cdot 12=36$.

(Problem proposed by A. Pesare)