语法与词汇 11.___program has been developed up to this date, but none of them are economical enough to be carried out now.
【正确答案】 D
【答案解析】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。