The word" premise" ( Line 3, Para. 1 ) probably means" ______ ".

A．it costs a large amount of money which is not affordable
B．the government worries about the quality of the water conservancy
C．it destroys the habitat of plants and animals of that area
D．the government becomes more and more ignorant

A．if dams were built in river basins, the downstream ecology remained the same
B．governments have to be responsible for the changes caused by the constructions of dams
C．the results of controlling water flows by engineers can be well forecasted
D．a dam can bring economical advantages for almost a century