Why will the following code not compile?

namespace ns1
{
void print();
void display1(){};
}

namespace ns2
{
void print();
void display2(){};
}

int main()
{
using namespace ns1;
using namespace ns2;
display1();
display2();
print();
return 0;
}
a. The call to print is ambiguous
b. We have not included the iostream library
c. We have not used namespace std
d. It will compile

a. The call to print is ambiguous