7. A. A fair cube die has the numbers $1,2,2,3,3,4$ on its six faces; another fair cube die has the numbers $1,3, 4, 5, 6, 8$ on its six faces. When these two dice are rolled simultaneously, the probability that the sum of the numbers on the top faces is 5 is
7. A. $\frac{1}{9}$.

Among the 36 possible outcomes, there are 4 pairs
$$
(1,4) 、(2,3) 、(2,3) 、(4,1)
$$

that sum to 5. Therefore, the probability that the sum of the numbers on the top faces is 5 is $\frac{4}{36}=\frac{1}{9}$.