3. Give a reason why
b. a river keep flowing even if there's a drought

The water flowing in a stream or river is fed by numerous smaller streams, which are fed by even smaller streams that collect water from melting snow or ice up in the mountains, from springs, and possibly from human wasterwater sources. The bottoms of rivers are also below the water table in many places, so there will always be water at that level.

separation of church and state? not sure what you mean.