Two discussions from different points of view may ______ each other.

A．existing technology fails to grow dry-land crops
B．only dozens of years later can they make profits
C．they are not encouraged and probably cannot make any profit
D．most of African countries belong to the third world

A．if dams were built in river basins, the downstream ecology were 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