The old lady has developed a ______ cough which cannot be cured completely in a short time.

A．eternal
B．external
C．internal
D．interval

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