Question

what color does bromothymol blue change to in the presence of co₂?
orangish - yellow
reddish - orange
bluish - purple
yellowish - green

Explanation:

Bromothymol blue is a pH - indicator. When carbon dioxide ($CO_2$) dissolves in water, it forms carbonic acid ($H_2CO_3$), which lowers the pH. Bromothymol blue turns orangish - yellow in acidic conditions.

Answer:

orangish - yellow