18. As shown in the figure, two identical containers are filled with liquids of the same density ρ = 1200 (kg/m³) and are connected by a communicating tube. A valve in the middle of the tube maintains the liquid height at h₁=15 (cm) on the left and h₂ = 7 (cm) on the right. After the valve is opened, the liquid flows until it comes to rest. Calculate the magnitude of the work done by gravity. [The cross-sectional area of both containers is 28.2 (cm²) and the gravitational acceleration is g = 9.8 m/s²]

(A) 1.6×10⁻² Joule
(B) 5.3×10⁻² Joule
(C) 2.6×10⁻² Joule
(D) 7.1×10⁻² Joule