Concept:An acidic gas dissolves in water to form an acidic solution, which lowers the pH.
Explanation:The pH changing from 7 to 6 means the water became slightly acidic.
In air, the gas that can cause this is carbon(IV) oxide,
CO2.
When
CO2 dissolves in water, it forms carbonic acid:
CO2+H2O→H2CO3Carbonic acid releases hydrogen ions,
H+, so the pH falls below 7.
Carbon(II) oxide, nitrogen, and oxygen do not produce acidic solutions in water.
Answer:A. carbon(IV)oxide