题目

 In ancient Egypt，the pharaoh(法老)treated the poor message runner like a prince when he arrived at the palace，if he brought good news. However，if the exhausted runner had the misfortune to bring the pharaoh unhappy news，his head was cut off. Shades of that spirit spread over today’s conversations. Once a friend and I packed up some peanut butter and sandwiches for an outing. As we walked light-heartedly out the door，picnic basket in hand，a smiling neighbor looked up at the sky and said，“Oh boy，bad day for a picnic. The weatherman says it’s going to rain.” 空气是人类宝贵的自然资源。下列有关空气的说法错误的是 A．液氮可用作制冷剂          B．氧气可用作火箭发射的燃料 C．氦气可用来填充探空气球    D．二氧化碳是植物进行光合作用必需的物质