Once , a plan should be carried out.A. makeB. makingC. madeD. to make

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A. make
B. making
(答案→)C. made
D. to make
解析：考查非谓语动词。句意：一旦制定，计划就应该执行。过去分词表示被动的，发生过的动作，和句子主语是动宾关系，plan是make的对象，故选C。