They talked in a very low voice ________make themselves ________ clearly.A.in order to; hearB.so as not to; hearC.in order not to; heardD.so that to; heard
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C解析本题要考虑两个知识点。so as to do和in order to do都可以表示目的，只不过so as to不用于句首，两个短语的否定形式都是在不定式符号to前加not或never. so that后面接的是从句。make oneself heard是过去分词作宾语补足语，意为“使自己让别人听见”。综上所述，选项D为正确