Some cities have passed laws that allow coal and oil______only if their sulfur content is low.
【正确答案】 D
【答案解析】解析：本题考查动词allow的用法。allow sb．to do sth．，“允许某人做某事”，所以A和C排除。煤和石油应该是被燃烧，应该用被动语态，故D为正确答案。