Water flows steadily through a horizontal pipe that narrows from a cross-section of $10\ \text{cm}^2$ to $5\ \text{cm}^2$. Two thin vertical tubes, open at the top, are fixed to the wide part and to the narrow part, as shown in the figure. The water in the tube over the wide part stands $10\ \text{cm}$ higher than in the tube over the narrow part. Taking $g = 10\ \text{m/s}^2$, the volume of water flowing through the pipe per second is closest to

Fig. 1: A horizontal pipe narrowing from 10 square centimetres to 5 square centimetres, with water flowing. Vertical tubes on the wide and narrow parts show water levels 10 cm apart.