________ the mountains was hard work for them but as they looked around, they could not help screaming.

A．Climb

B．To climb

C．For climbing

D．They climbed

有如下的程序：
#include ＜cstring.h＞
#include ＜iostream. h＞
using namespace std;
class MyString public:
MyString(const char*s);
～MyString() delete[]data;
protected:
unsigned len;
char*data;
;
MyString::MyString(const char *s) len=strlen(s);
data=new char[len+ 1];
strcpy(data, s);int main() MyString a("C++ Programing");
MyString b(a);
return0；在运行上面的程序时出错，出错的原因是( )。

A．构造函数的实参不允许是本类的对象

B．没有定义实现深层复制(深拷贝)的复制构造函数

C．构造对象a时实参与形参类型不符

D．系统不能生成默认的复制构造函数