C++ cast operator overloading and polymorhism

I'm puzzled with this behavior of C++:

struct A {
   virtual void print() const { printf("a\n"); }
};

struct B : public A {
   virtual void print() const { printf("b\n"); }
};

struct C {
   operator B() { return B(); }
};

void print(const A& a) {
   a.print();
}

int main() {
   C c;
   print(c);
}

So, the quiz is, what is the output of the program - a or b? Well, the answer is a. But why?