When the dolpin ( ) by sea ,it ( ) A discovered had killed B was discovered had been killedC discovered had been killed D was discovered had killed 为什么?

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When the dolpin ( ) by sea ,it ( ) A discovered had killed B was discovered had been killed
C discovered had been killed D was discovered had killed
为什么?

选 B was discovered had been killed
都是被动语态
首先是被发现 后是 已经被杀 用过去完成时