Acid precipitation forms when fossil fuels are ______.

• A. Burned
• B. Burning
• C. Burn
• D. Burnt

Answer: (A)

Acid precipitation forms when fossil fuels are burned. The sentence uses a passive construction, so the correct word is the past participle form of burn. Burned is the standard American English past participle, yielding “fossil fuels are burned” to indicate a completed action that releases pollutants leading to acid rain. The other forms don’t fit the grammar: burning describes ongoing action, which changes the meaning; burn is the base form and doesn’t fit after “are”; burnt is the British variant and, while acceptable in some contexts, is less common in American usage.