Sentence Correction

The reasons for yesterday's sharp rise in the stock market were much the same as that behind last week's rally: a surge in the economy to a 5.6 percent annual growth rate and improved corporate earnings balanced by the lack of signs of inflationary pressure.

A. that
B. those
C. what was
D. for that
E. they were

Would C be right if "what was" is changed into "what were"? Thanks, the correct answer is B.

The reasons for yesterday's sharp rise in the stock market were much the same as that behind last week's rally.

This question asks whether you can spot "parallelism".
"those" is needed because it refers to "the reasons"(plurals)

The reasons for yesterdays's sharp rise.....
those(=the reasons) behind last week's rally.