I would have told him the answer had it been possible, but I ________ so busy then．
A.had beenB.wereC.wasD.would be在线课程C
本题是个表示转折关系的并列句，前一分句是与过去事实不符的虚拟语气条件句，but以后为过去事实，动词要用一般过去时，故应选C项。