According to Some Researchers, the Reason Studies Don't Always Find

According to some researchers, the reason studies don't always find a relationship between Type A behavior and health problems may be that

A) only some of the behaviors within the Type A pattern are related to health problems.
B) researchers are doing a better job of measuring Type A today.
C) people have become aware of the causes of heart attacks in recent years and have taken actions to prevent them.
D) people today are working to reduce stress in their lives.