比较四个选项的意思：

It’s a pity：真遗憾！

I couldn’t agree more：我完全赞成。

Forget it：休想。不可能。没关系。

I can hardly thank you enough：感谢不尽。非常感谢。

你把四个选项放进去试试，就知道只有I can hardly thank you enough最合适。

—I worked on your car the whole night. How is it running? 我昨天整晚都在修你的车。它现在怎么样？

—It is running great! I can hardly thank you enough. You were such a big help! 它运转良好！太感谢你了。你真是帮了我的大忙。