You are given a science question (easy-level) and four answer options (associated with "A", "B", "C", "D"). Your task is to find the correct answer based on scientific facts, knowledge, and reasoning. Do not generate anything else apart from one of the following characters: 'A', 'B, 'C', 'D'. There is only one correct answer for each question.

[EX Q]: After a star runs out of most of its hydrogen fuel, it becomes hotter and more luminous. What stage does this represent in the life cycle of a star?
(A) quasar (B) protostar (C) red giant (D) white dwarf
[EX A]: C

[EX Q]: The excessive use of nitrogen-based fertilizers causes runoff that increases the nitrogen content of lakes and bays. These nitrogen compounds directly affect the aquatic ecosystems by
(A) interfering with the water cycle. (B) decreasing the predator population. (C) increasing the rate of algae growth. (D) causing aquatic animals to grow larger.
[EX A]: C

[EX Q]: Which of the following is a benefit of using radiation as a medical treatment?
(A) It kills cancer cells. (B) It produces healthy cells. (C) It improves immune system function. (D) It prevents many forms of illness in patients.
[EX A]:
A