企源知识库

Some cities have passed laws that allow coal and oil __________only if their sulfur content is low.
A、burning
B、to burn
C、being burned
D、to be burned
【正确答案】：D
【题目解析】：一些城市已通过立法：只有当煤和石油的副产品很少时，才允许其作为燃料使用。