Hydroelectric power is produced by
a. falling water that turns a turbine.
b. tides that pour through a dam barrier.
c. hot water that comes from deep underground.
d. electric current that flows across a dam.

Hydro electric power is the process of  production of electricity from the mechanical energy of water.

In the process of generation of  hydro electricity ,large dams are constructed  in which the water is stored.The water present at top of the dam has potential energy.The water is driven downward  with high pressure. During the falling of water,the potential energy of the water is converted into equivalent kinetic energy. The water coming down with a heavy speed is allowed to fall on a turbine which is connected to a generator.

The mechanical energy of water drives the turbine and electricity is generated due to the electromagnetic induction .

Hence the option A is right i.e falling water that turns the turbine.

the old price per ounce is c/4.

the new price per ounce is c/3.6.

the ratio of the old price to the new one is

(c/4)/(c/3.6) = (c/c)×(3.6/4) = 0.9 . . = 9/10 . . "nine tenths" in either form