Sulfur dioxide, which smells like rotten eggs, is formed in smokestacks from sulfur and oxygen, two of the major elements consumed in coal smelting, react with each other.

a) is formed in smokestacks from

b) is formed in smokestacks when

c) is formed in smokestacks, and when

d) formed in smokestacks when

e) formed in smokestacks from

OA B

Source: Veritas Prep