In the Featured Study by Dutton & Aron, the Authors

In the Featured Study by Dutton & Aron, the authors hypothesized that an attractive female (who was a research confederate) would be perceived as more attractive by males on "fear-arousing" bridge, than by males on a "control" bridge. Further, if a man crossed the "fear-arousing" bridge prior to meeting a female, he would be more likely to phoned her later. In this study, what is the independent variable?

A) whether the male subjects phoned the female confederate
B) whether a male subject crossed a bridge, or not
C) the 'fear-arousing' bridge
D) the baseline attractiveness of the female confederate