A researcher has conducted an experiment in which she has given participants a placebo, 10mg, and 20mg of a drug. She then measured their anxiety, blood pressure, and heart rate in response to the drug. How many levels does this experiment’s independent variable have?

Three — "placebo (0 mg), 10mg, and 20mg of a drug."

To determine the number of levels in this experiment's independent variable, we need to identify the different conditions or treatments that participants are exposed to. In this case, the independent variable is the dosage of the drug administered to the participants. The researcher has given three different doses: a placebo, 10mg, and 20mg.

Therefore, this experiment has three levels for its independent variable: placebo, 10mg, and 20mg.