Answer:
Because it produce weak acid and acid turns blue lithmus paper to red.
Step-by-step explanation:
Sulphur dioxide SO2 react to added water to produce acidic product.
When sulphur dioxide react with water, it produce sulphurous acid H2SO3 which is a weak acid.
One of the properties of acid is that they turn blue lithmus papper to red, because the product is an acid, blue lithmus papper turns to red.