Which Situation Describes Potential Observer Bias in an Experimental Process

Which situation describes potential observer bias in an experimental process?

A) Dr. Jones is evaluating cancer patients for their responses to a new therapeutic drug. She knows which patients are receiving the placebo and which are receiving the drug.
B) Mr. Bromley is conducting a survey of weight loss for his professor. He asks each student in the study the same questions.
C) Ms. Bradley is an outside consultant who is conducting a health and wellness survey for a pharmaceutical company. She does not know the name of the company nor does she know the name of the drug being tested during the survey.
D) Dr. Postgate is analyzing biopsy samples from rats that have been given either a placebo or an experimental drug believed to reduce inflammation. Each sample is identified by a code number so that Dr. Postgate cannot tell which treatment the rats received.