The correct options are
A A barrier across running water
B Classified based on structure, material and height
A dam is a barrier across flowing water to obstruct, direct or hamper its flow by creating a reservoir, lake or an impoundment. These dams are usually built to retain the rivers and the rainwater that could be used to irrigate the agricultural fields.
Dams are often classified on the basis of structure and materials used. The classification includes timber dams, embankment dams and masonry dams. Whereas, on the basis of height, dams are commonly classified as low dams, medium dams, and high dams.