What is cyclic electron flow used for during the light reactions of photosynthesis?

a neutral ph of 7 and normal organism temperature is needed for a certain enzyme to be effective.  

what would happen to the enzyme if the ph and/or temperature fluctuate greatly from the norm?  

a.the enzyme’s hydrogen bonds will break.  

b.the activation energy will increase.  

c.more product will be formed.

d.the ideal temperature and ph for the enzyme function will decrease.  

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.