选C没错，表示写信的事不想让同事看到。但对不能选D的解释不对。可以用过去完成时，但不能用had done，had done 意思不够明确。是表述的问题，不是时态问题。假如写成：...because she didn't want her colleagues to know that she had decided to quit her job 或...because she didn't want her colleagues to know that she had decided to resign 这样就可以了。