Identify the manipulated variable and the responding variable for the study.

Scientists are working hard to learn about causes of cancer. It is known that in general terms, cancer-causing agents produce mutations in a variety of genes that control cell reproduction.

Cancer can be caused by organisms. For example, when some viruses infect a human cell, the virus DNA inserts itself randomly into the DNA of human cells.

Of all the cancer-causing agents to which humans are commonly exposed, tobacco smoke appear to cause the greatest harm. Tobacco is directly responsible for 30% of all cancer deaths in North America. Heavy smoking increases the likelihood of getting lung cancer by 2000%. Passive (second-hand) smoke is less likely to cause cancer. It is about as harmful as all other forms of air pollution.

A researcher wants to develop a study to help determine what percentage of smokers will die from cancer caused by tobacco and its smoke. The researcher gathers two groups of subjects and observes them over a long period of time.

Identify the manipulated variable and the responding variable for the study.