Why is water vapor part of both the hydrosphere and the atmosphere ?

Water vapor is part of hydrosphere and the atmosphere because they both evaporate when rain falls down so say I’m making boiled eggs and there’s water vapor coming out of the pot that means that the water is evaporating it’s not smoke just so u know I learned this in 5th grade

Iam not sure but to me it with be a or dsorry if i was late! hope this ! : )if you really want me to find out the answer that is correct tell me and i will do more research but the a or d makes sense to me.