Which of the following is not an example of systematic error in an observational study?

A) A cross-sectional study recruits participants that are willing to sign up outside of a major university and meet the inclusion and exclusion criteria to take part in the survey relating unsafe sex habits to STIs.

B) A researcher is interested in the relationship between coffee drinking and lung cancer, and after careful multivariate linear regression modeling determines that a significant percentage of the relationship is due to another variable, cigarette smoking.

C) An observational study recruits participants for a study looking at Alzheimer's disease due to exposure to industrial hazards by asking participants to recall their exposure over the past 10 years.

D) Data-entry specialist responsible for adding in fasting glucose levels to a database accidentally skipped an observation during the input phase of data cleaning.