Which of these effects on digestion is not produced by the parasympathetic nervous system?

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds