题目

 Don’t be angry with us. We ______ with you together, but we were busy with our final    reports when you called in.   A. would eat out                                           B. must have eaten out     C. should eat out                                           D. could have eaten out 答案：D元素X的原子核外第三层上有3个电子，元素Y的负二价离子离子核外有10个电子，则这两种元素形成化合物的化学式为（ ）A．XY2B．X2YC．X2Y3D．X3Y2