题目

 What worried the child most was _______ to visit his mother in the hospital. A. his not allowing        B. his not being allowed      C. his being not allowed     D. having not been allowed 答案：B工业上常用乙炔燃烧产生的高温火焰来切割金属．若用R表示乙炔，其燃烧的化学方程式：2R+5O2 点燃 . 4CO2+2H2O，则乙炔的化学式是（　　）A．C2H4B．C2H2C．CH3COOHD．C2H6