Please help. answer the question below

I think it is temperature because that is a key to help growing kelp

its either co2 or o2

Explanation:

i cant remember specifics but i know for a fact water clarity and temperature isnt going to affect kelp. that thing wont die if u lit it on fire

b. the carbon cycle ensures living things have a source of hydrogen

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.