Answer: C) other variables (for example, cheaper gasoline or the season of the year during which the change was instituted) may also be affecting accident rates.
Explanation: Before making conclusion on causality, several other independent variables should be considered before arriving at a reasonable conclusion. Accident causing factors are numerous which could include, season of the year in which the change in speed limit was made, this may have a say in the weather condition which could impede vision, which could also result in accident , gasoline type, and other events which occurred within the speed change period which could have impacted the accident rate should be considered before arriving at a conclusion.