5. The ground floor of a hotel has 5 fewer rooms than the second floor. A tour group has 48 people. If all are arranged to stay on the ground floor, each room can accommodate 4 people, but there are not enough rooms; if each room accommodates 5 people, some rooms are not fully occupied. If all are arranged to stay on the second floor, each room can accommodate 3 people, but there are not enough rooms; if each room accommodates 4 people, some rooms are not fully occupied: The ground floor of this hotel has ( ) rooms.
(A) 9
(B) 10
(C) 11
(D) 12
5. (B).

Let the number of guest rooms on the ground floor be $x$, then the number of rooms on the second floor is $x+5$. According to the problem, we have the system of inequalities
$$
\left\{\begin{array}{l}
\frac{48}{5}<x<\frac{48}{4}, \\
\frac{48}{4}<x+5<\frac{48}{3} .
\end{array}\right.
$$

which simplifies to $\left\{\begin{array}{l}9.6<x<12, \\ 7<x<11\end{array} \Rightarrow 9.6<x<11\right.$.
Solving this, we get $x=10$.