7. B. 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 7 is
7. B. $\frac{1}{6}$.

As above, the probability that the sum of the numbers on the top faces is 7 is $\frac{6}{36}=\frac{1}{6}$.