Strayer and Colleagues Wanted to Establish If There Was a Causal

Strayer and colleagues wanted to establish if there was a causal relationship between cell phone use while driving, traffic density, and increased risk of vehicular collision. They found evidence of an interaction because:

A) cell phone use did not affect reaction time in regardless of traffic density.
B) cell phone use led to an increase in braking reaction time in the low traffic density condition.
C) cell phone use led to an increase in braking reaction time only in the high traffic density condition.
D) cell phone use led to an increase in braking reaction time in both the low and high traffic conditions.