___program has been developed up to this date, but none of them are economical enough to be carried out now.

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问题 ___program has been developed up to this date, but none of them are economical enough to be carried out now.

选项 A、All
B、Several
C、Either
D、Many a

答案D

解析 题干属于but连接的并列句。前半旬的谓语动词为单数,且后半句中的them指代的是program,表示的是复数的概念,因此用many a修饰名词,故选D。
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