问题
单项选择题
汽车发动机在燃烧时会产生苯，而苯是一种致癌物质。为了应对人类面临的这一威胁，环境保护主义者建议用甲醇来替代汽油，其理由是：燃烧单位数量的甲醇所产生的苯，只是汽油的十分之一。 以下哪项如果为真，能构成对上述环境保护主义者建议的质疑 Ⅰ．燃烧甲醇时会产生甲醛，而甲醛同样是一种致癌物质。 Ⅱ．燃烧单位数量的甲醇所产生的能量，大约只是汽油的十分之一。 Ⅲ．就应用前景而言，甲醇的价格大约只是汽油的十分之一。
A．仅仅Ⅰ。
B．仅仅Ⅱ。
C．仅仅Ⅲ。
D．仅仅Ⅰ和Ⅱ。
E．Ⅰ、Ⅱ和Ⅲ。
答案
参考答案：D
解析：选项Ⅰ能削弱题干，因为用甲醇来替代汽油虽然所产生的苯比较少，但产生的甲醛同样是致癌物质，所以不可取。选项Ⅱ也能削弱题干，因为燃烧甲醇虽然产生的苯较少，但所产生的能量也少，所以要产生同样的能量，就会产生同样多的苯。选项Ⅲ与题干无直接关系，不能削弱题干。
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