Linker error with template classes

I have a strange error while compiling template classes. This is my minimal example:

#include <iostream>

using namespace std;

template<class T>
class A{
        T element;
        protected:
                virtual bool empty() const noexcept = 0;

};

template<class T>
class B : public A<T> {
        public:
                bool tryOP(T &element) const {
                        if (A<T>::empty())
                                return false;
                        else
                        {
                                std::cout << "Operation" << std::endl;
                                return true;
                        }
                }
};

template <class T>
class C : public B<T> {

        private:

                bool empty() const noexcept override{
                        return true;
                }
};

int main(){
        int n = 0;
        C<int> c;
        c.tryOP(n);
}

The error:

/tmp/ccJvMBCZ.o: In function B::tryOP(int&) const: A.cpp:(.text._ZNK1BIiE5tryOPERi[_ZNK1BIiE5tryOPERi]+0x18): undefined reference to `A::empty() const' collect2: error: ld returned 1 exit status

Why undefined reference to empty()? It's right there!