6. (10 points) Pools $A$ and $B$ are both rectangular with a length of 3 meters, a width of 2 meters, and a depth of 1.2 meters. Valve 1 is used to fill Pool $A$, and it takes 18 minutes to fill an empty Pool $A$. Valve 2 is used to transfer water from Pool $A$ to Pool $B$, and it takes 24 minutes to transfer all the water from a full Pool $A$ to Pool $B$. If both Valve 1 and Valve 2 are opened simultaneously, then when the water depth in Pool $A$ is 0.4 meters, the volume of water in Pool $B$ is (   ) cubic meters.
A. 0.9
B. 1.8
C. 3.6
D. 7.2
【Analysis】According to the problem, let the capacity of pools $A$ and $B$ be “1”. The water inflow efficiency of Pool $A$ through Valve 1 is $\frac{1}{18}$ per minute, and the water inflow efficiency of Pool $B$ through Valve 2 is $\frac{1}{24}$ per minute. The water outflow efficiency of Pool $A$ is also $\frac{1}{24}$ per minute. When Valves 1 and 2 are opened simultaneously, the water inflow efficiency of Pool $A$ is $\frac{1}{18}-\frac{1}{24}=\frac{1}{72}$ per minute, and the water inflow efficiency of Pool $B$ is $\frac{1}{24}$ per minute. If the water level in Pool $A$ is 0.4 meters, then the water volume in Pool $A$ is $0.4 \div 1.2=\frac{1}{3}$. The time required is $\frac{1}{3} \div \frac{1}{72}=24$ minutes. The water volume in Pool $B$ is $24 \times \frac{1}{24}=1$, so the water volume in Pool $B$ is $3 \times 2 \times 1.2=7.2 \mathrm{~m}^{3}$.

【Solution】Solution: Let the capacity of pools $A$ and $B$ be “1”.
When Valves 1 and 2 are opened simultaneously, the water inflow efficiency of Pool $A$ is: $\frac{1}{18}-\frac{1}{24}=\frac{1}{72}$. If the water level in Pool $A$ is 0.4 meters, then the water volume in Pool $A$ is: $0.4 \div 1.2=\frac{1}{3}$. The time required is: $\frac{1}{3} \div \frac{1}{72}=24$ minutes.
The water volume in Pool $B$ is: $24 \times \frac{1}{24}=1$.
So the water volume in Pool $B$ is: $3 \times 2 \times 1.2=7.2$ (cubic meters).
Therefore, the answer is: $D$.