Independent variable: different amounts of second-hand smoke
Explanation:
The independent variable in an experiment is the variable that the experimenter changes or manipulates in order to bring about a measurable response.
In this question, a test is being carried out to determine the health effects of different amounts of second-hand smoke on one-year-old hamsters. Based on this, the independent/manipulated variable is the DIFFERENT AMOUNTS OF SECOND-HAND SMOKE because it is the variable being changed.
