A Study Evaluated the Relationship Between the Common Cold and the Number

A study evaluated the relationship between the common cold and the number of cigarettes smoked per day.  The research was conducted among fourth-year medical students and sponsored by the public health department of the medical school.  Medical students with symptoms of common cold were asked to fill out a questionnaire about their smoking status, number of packs smoked per day, and duration of smoking.  Which of the following factors would most likely invalidate the findings of this study?

A) Admission rate bias
B) Lead-time bias
C) Nonresponse bias
D) Regression to the mean
E) Response bias