Question about respiration reaction?

Isn't this question invalid? A question on my sisters biology test was as follows: Which element is not used in photosynthesis or respiration? 4 answer choices: oxygen, carbon, nitrogen, hydrogen. Aren't all of them used at some point? If I recall denitrification is a form of respiration done by some microbes. NO3-+2e-+2H -----> NO2- +H2O is the reduction of nitrate to nitride (the cycle can continue until N gas is released, and it is broken down completely) if I remember my bio class some bacteria can do it (excuse misspellings) pseudomonas stutzen, Arigonsia, rhodobauter sphanelis. Someone please confirm or disprove my theory that all are (can be) used.