How Do Run-Of-The-River Hydroelectric Systems Differ from Giant Dams

How do run-of-the-river hydroelectric systems differ from giant dams?

A) They don't block all of the water flowing down a channel and direct only some of it through a turbine.
B) They literally "run" the river and take over the entire ecosystem.
C) They flood an area to create a reservoir.
D) They do not differ; they work the same way.
E) They are built close to shorelines,whereas giant dams can be built only farther inland.