Melissa ate the cake I had baked.
-->I had baked the cake before Melissa ate it.
時間點是Melissa吃掉那塊蛋糕的時候。
ate (過去) 改成 eat (現在)
答案就是A
過去完成式也可以用作 "過去的過去"
Melissa ate the cake that I ______. (..-阿摩線上測驗