In 1929 relatively small declines in the market ruined many [U]speculators having bought on margin; they had to sell, and [/U]their selling pushed other investors to the brink.
A.
B.speculators who had bought on margin; having had to sell,
C.speculators who had bought on margin; they had to sell, and
D.speculators, those who had bought on margin; these speculators had to sell, and
E.speculators, who, having bought on margin and having to sell,