In a Study Designed to Identify Factors Involved in Helping

In a study designed to identify factors involved in helping behavior, a man on a crowded bus clutches his chest and falls to the floor. In one of the conditions of the study the man is clean shaven and wearing a suit; in the other condition he has a scraggly beard and is wearing a dirty t-shirt and jeans. The amount of time it takes for someone to help the man is recorded. In this example the independent variable is

A) the amount of time it takes someone to help.
B) the appearance of the man.
C) the participants in the study.
D) how crowded the bus is.