Question

which hold water until it is needed?
dams
reservoirs
aqueducts
valleys

Explanation:

Dams are structures that block water flow to store it, while reservoirs are the storage basins created by dams. However, the question asks what holds water until needed: reservoirs are the specific bodies that hold the water, dams are the structures that enable this storage. Aqueducts transport water, and valleys are landforms that may naturally carry water but do not intentionally store it for later use.

Answer:

B. reservoirs