The correct options are
A It takes a lot of time to build dams.
B Villages and towns submerge due to backwater storage.
C Shortage of water down the river.
Dam takes a long time from planning to construction and for them being fully functional to generate electricity. The stored water behind the dam can submerge villages, cities and fertile lands nearby. The amount of water down the river reduces due to construction of dams causing water shortage to people who live downstream. Hydroelectric power plant does not produce any significant waste.