设有以下定义和程序:#include<iostream.h>class A1{public: void show1() { cout<<"class A1"<<endl; } };class A2 : public A1{public: void show2() { cout<<"class A2"<<end1 }};class A3 : protected A2{public: void show3() { cout<<"class A1"<<endl; }};void main(){ A1 obj1; A2 obj2; A3 obj3;} 则以下不合语法的调用语句是
A.obj1. showl();
B.obj2. showl();
C.obj3. showl();
D.obj2. show2();