I took my cue in a moment, and did the only rational thing there was to do.

I reached the note towards him, and said, carelessly: Give me the change, please.

這里的and did the only rational thing there was to do. 中：there was to do 不知道。

若是there was， 那是定語從句。

there was to do 是不是也是定語從句？若是，如何理解呢？do代表了什么呢？

there do 麻煩老師剖析一下，謝謝！

the only rational thing （that）there was to do.

省略的that指代先行詞，并在定語從句作主語，to do為that的定語，動賓關系。比較：

there was one thing to do 不定式為定語，表示動賓關系。