题目

____________ the problem for many years, he announced that we are now facing the biggest environmental problem of our time．A．Observed                                                 B．Observing                  C．Having observed                                       D．Having been observed 答案：C标准大气压下，水的沸点是100℃，若减小气压，则水的沸点    100℃（填“低于”，“等于”或“高于”）