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C 首先要弄清in order to与so as to的区别
in order to=so as to=介词to
但是in order to 只能表示目的,其位置在句首或句中皆可
,否定句结构用in order not to,比 so as to正式；so as to既可表示目的,又可表示结果,其位置一般在句中,否定结构用so as not to ；so. as to只能表示结果,否定结构用so. as not to
再看寄信用被动 所以是C