A group of researchers investigated the effects of a drug called pressure drop y on lowering blood pressure in a group of men between the ages of 50 and 70 years old. The researchers did the following experiment and obtained the indicated results: One group of 150 men took a tablet containing pressure drop Y for 3 weeks -95 of these men experienced decreased blood pressure by at least 10%. Another group of 150 men were given a tablet with no added pressure drop Y for 3 weeks - 10 of these men experienced decreased their blood pressure by 10%. What is the independent variable in this experiment?
a) the group of men that received the pills with no added pressure drop y
b) the 3 week time period and sample size of 150
c) amount of pressure drop y
d) the age and gender of the participants
e) blood pressure