题目内容：
Natural gas can serve as an______to coal and oil because it is more environmentally friendly.
A.option
B.extra
C.addition
D.alternative
参考答案：
答案解析：
选择题：Natural gas can serve as an______to coal and oil because it is more environmenta
- 题目分类：成考（专升本）
- 题目类型：选择题
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