Most freshwater use is
a. for irrigation
b. for industry and electrical power
c. to supply the public with drinking water
d. for swimming pools

Most freshwater use is - For industry and electrical power                    

The freshwater or running water is also used for irrigation and to supply people with water for everyday uses. But this is only about 30%.

So, the rest 70% of the fresh surface water is used in power plants to cool off machinery. This used water is again recycled and used or sometimes it is returned to its source.      

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.