문제 내용: 다음 글의 요지로 가장 적절한 것은?
선택지 1: ① Viruses demonstrate self-sufficiency of biological beings.
선택지 2: ② Biologists should do everything to keep plants virus-free.
선택지 3: ③ The principle of symbiosis cannot be applied to infected plants.
선택지 4: ④ Viruses sometimes do their hosts good, rather than harming them.