6. 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, which can fill an empty Pool A in 18 minutes; Valve 2 is used to transfer water from Pool A to Pool B, which can transfer a full Pool A to Pool B in 24 minutes. If Valves 1 and 2 are opened simultaneously, then when the water depth in Pool A is 0.4 meters, Pool B has ( ) cubic meters of water.
A. 0.9
B. 1.8
C. 3.6
D. 7.2
Analysis: In and out water project problem.
Let the capacity of water pool A and B be "1", the water inflow efficiency of valve 1 in pool A per minute be $\frac{1}{18}$, the water inflow efficiency of valve 2 in pool B per minute be $\frac{1}{24}$, and the water outflow efficiency of pool A per minute be $\frac{1}{24}$. When valves 1 and 2 are opened simultaneously, the water inflow efficiency of pool A per minute is $\frac{1}{18}-\frac{1}{24}=\frac{1}{72}$, and the water inflow efficiency of pool B per minute is $\frac{1}{24}$. If the water depth in pool A is 0.4 meters, then the water inflow in pool A is $0.4 \div 1.2=\frac{1}{3}$, which requires $\frac{1}{3} \div \frac{1}{72}=24$ minutes. The water inflow in pool B is $24 \times \frac{1}{24}=1$, so the water volume in pool B is $3 \times 2 \times 1.2=7.2 \mathrm{~m}^{3}$. The answer is D.