In a controversial study to determine the effectiveness of AZT, a group of HIV-positive pregnant women were

Question:

In a controversial study to determine the effectiveness of AZT, a group of HIV-positive pregnant women were randomly assigned to get either AZT or a placebo. Some of the babies born to these women were HIV-positive, while others were not.
(a) What is the explanatory variable?
(b) What is the response variable?
(c) What are the experimental units?