Final answer:
The independent variable is the exposure to X-rays, which is being manipulated by the researchers, and the dependent variable is the number of offspring with birth defects, which is the outcome measured.
Step-by-step explanation:
In the context of the study involving white rats to determine the relationship between offspring birth defects and exposure to X-rays, the independent variable (IV) is the number of minutes of exposure to X-rays by the mother rats. This is because the independent variable is what the researchers are manipulating to observe its effect on the outcome. On the other hand, the dependent variable (DV) is the number of offspring born with birth defects, which is the outcome that the researchers expect to change based on the level of X-ray exposure. Essentially, the dependent variable is expected to depend on the independent variable.