单选题：设有以下定义程序：#include<iostream.h>class A1 { public: void show1()

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