题目

The research is so designed that once _____ nothing can be done to change it. A. begins                           B. having begun          C. beginning               D. begun 答案：D下列混合气体，遇明火不会爆炸的是（ ）A．管道煤气和空气B．氧气和氮气C．氢气和空气D．氧气和氢气