答案：C

题目分析：句意:如果我昨天来的话我就能帮助那位老人。这是省略if的条件状语从句，并且是虚拟语气，从句中有过去的时间状语，所以对过去的虚拟谓语应该用had done，省略if时，把had提到主语的前面，故选C。

点评：本题难度适中。虚拟语气是高中阶段的重要语法项目，需要考生牢记各种情况的虚拟语气的谓语动词的形式，并要有分析理解语境的能力。

即学即练：______ smoking, he would not have got cancer in the lung.

A. Was he given up B. Had he given up

C. Did he give D. If he gave up

解析：B。由于主句谓语是would not have got，这表明是对过去事实作出的假设，所以从句谓语应该是 had given up，而不是像D项那样用一般过去时。另外，当虚拟条件句中有 had, should, were 等词时，通常可以省略 if，并将 had, should, were 提前置于句首。