题目

   That year, in the local school, there was a new math teacher, as well as some new pupils. One of the new kids, Mario, was the stupidest child anyone had ever seen. It made no difference how quickly or how slowly they tried explaining numbers to him; he would always end up saying something extremely dumb(愚蠢的). Like two plus two was five, seven times three was twenty-seven, or a triangle had thirty corners.    Before this boy arrived, math lessons had been the most boring of all. Now they were great fun. Encouraged by the new teacher, the children would listen to the pieces 下列物质只含有离子键的是( )A. KOH B. NO2 C. H2O2 D. MgC12