You have a weight loss study going on that asks everyone (all of those in the control and treatment groups) to record the beverages/foods (by amount) they consume everyday, for 6 weeks. At the end of the study both the control group mean and the treatment group mean indicate a strong reduction in calorie consumption. What is going on here?

a- this is an example of random error - it will not change the mean

b- this is an example of bias created by the Hawthorne effect

c- this is an example of end aversion

d- this is an example of systematic error - it will change the mean

I need typed answer