oxygen nitrogen carbon dioxide carbon monoxide
I gave my students this multiple choice question: Fertilizer from an agricultural area runs off into a river. The river carries the nutrients from this fertilizer and deposits them into an ocean bay. After the nutrients enter the bay, scientists monitoring the water would most likely see a decrease in which of these dissolved gases?

Only one answer is correct among these 4 choices:
- oxygen
- nitrogen
- carbon dioxide
- carbon monoxide

Could you tell me which one is correct?

oxygen