Read the following article and answer the question. Article: The relationship between exercise and cancer has long both intrigued and puzzled oncologists and exercise physiologists. Exercise is strongly associated with lowered risks for many types of cancer. At the same time, exercise involves biological stress, which typically leads to a short-term increase in inflammation which can contribute to higher risks for many cancers. Now, a new study in mice may offer some clues into the exercise-cancer paradox. It suggests that exercise may change how the immune system deals with cancer by boosting adrenaline , certain immune cells and other chemicals that, together, can reduce the severity of cancer or fight it off altogether. To try to better understand how exercise can both elevate inflammation and simultaneously protect the body against cancer, scientists at the University of Copenhagen in Denmark and other institutions decided to closely examine what happens inside mice at high risk for the disease. So, for the new study, they began by gathering a group of adult lab mice. These animals generally like to run. The scientists then implanted melanoma skin cancer cells into the mice before providing half of them with running wheels in their cages while the other animals remained sedentary. After four weeks, far fewer of the runners had developed full-blown melanoma than the sedentary mice and those that had been diagnosed with the disease showed fewer and smaller lesions. They drew blood from both the exercising and sedentary animals and cells from any tumors in both groups. As expected, they found much higher levels of the hormone adrenaline in the blood of the exercising animals, especially right after they had been working out on the wheels but also at other times of the day. The body releases adrenaline in response to almost any type of stressful experience, including exercise. They also found higher levels of interleukin-6 in the blood of the runners. This is a substance that is released by working muscles and is believed to both increase and decrease inflammation... Question: According to the study, we can infer the fundamental substance to fight the cancer off is _ . Answer:
Available choices:
(a). adrenaline.
(b). interleukin-6.
(c). natural killer cells.
(d). genes.
(c).