题目

Tiredness, coughing, a runny nose and a sore throat—among all sicknesses there is probably none more common than the flu (流感), which we all get now and then. However, bird flu is a completely different story.In 2003, the H5N1 bird flu swept across 15 countries, including China, with sufferers reporting chest pain, difficulty breathing, fever and severe coughing. More than 600 people were infected, and about 60 percent died. Now, another type of bird flu hit Shanghai and three neighboring provinces, and this time the virus is called H7N9. By the afternoon of April 11, the new virus had ta实验小学兴趣小组到野外进行探索活动，遇到一棵很大的树。于是他们用一根长30米的绳子去围树干，围了4圈，还剩2米。这棵大树的树干的周长是多少米?