Question

When she arrived,she found that the letters____(take) away

Answer 1

had been taken.

本句话意思：她到的时候，她发现信件已经被带走了。

信件是被带走，所以应该用被动，加之take away这个动作发生在arrived的之前，所以是过去的过去，应该用过去完成时。
综上，答案：had been taken.

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Answer 2

taken

信是被拿走了，所以用过去分词，表示被动完成的动作

Answer 3

has been taken
当她到达时，信已经被拿走了。现在完成时

Answer 4

had been taken, 这个动作发生在found主语动作之前，所以应该用过去完成时，信被带走是被动式

Answer 5

had been taken 发生在found主语动作之前，所以应该用过去完成时，信被带走是被动式

Answer 6

had been taken