Final answer:
Liters (B) is the correct unit when measuring the volume of a pool in the metric system, although for large volumes like a pool, cubic meters might also be used.
Step-by-step explanation:
The unit you use when measuring a volume of a pool is liters (B). While a pool is a large container of water, the volume is best measured in liters or, for very large volumes like those of a pool, in cubic meters (m³). The reason we use liters or cubic meters instead of meters is because liters are a unit of volume. In the context of the SI (or metric) system, volume is a derived unit and a liter is defined as the volume of a cube that is 10 centimeters (1 decimeter) on each side. Hence, a liter is equal to 1 cubic decimeter (1 dm³) or 1000 cubic centimeters (1000 cm³).