Question

1. which of these would happen first if algae stopped growing in a pond? * 1 point

a. fish would die.
b. there would be less carbon dioxide.
c. there would be less oxygen.
d. plants would die.

Explanation:

Algae perform photosynthesis, producing oxygen ($\ce{O_2}$) and consuming carbon dioxide ($\ce{CO_2}$). If algae stop growing, their photosynthesis stops. The first direct effect is less oxygen production (so less oxygen in the pond). Fish dying (a) or plants dying (d) would be subsequent effects. Less carbon dioxide (b) is incorrect as algae consume $\ce{CO_2}$, so stopping growth would lead to less consumption, meaning more $\ce{CO_2}$, not less.

Answer:

c. There would be less oxygen.