题目

阅读理解 The forest in Senegal, a country in western Africa, is full of the chimps' usual noises. Suddenly dogs bark. Larger male chimps drop from the trees to face the threat while the others climb to safety. Then the dogs' young human masters appear. One mother chimp with a tiny baby tries to run. The dogs attack and separate them. The two teenage boys quickly catch the baby chimp. But they don't act out of sympathy — they save the baby so they can sell it. After the teenagers return to their hometown, they visit a man who is said to be very interested in chimps. When they ask the man,二氧化硫是形成酸雨的罪魁祸首．下列可作为检测空气中二氧化硫“指示植物”的是（ ）A．藻类植物B．苔藓植物C．蕨类植物D．裸子植物