Solve this Error unresolved external symbol LNK2001

Question

I got an error as I have mentioned in the title.

Below is the code

#include <iostream>

using namespace std;

class SavingsAccount {
private:
 double savingsBalance;
 static double AnnualInterestRate;
public:

 static void ModifyInterestRate(double rate) {
  AnnualInterestRate = rate;  
 }

 double calculateMonthlyInterest(double bal) {
  double total, newbal;
  total = (bal * AnnualInterestRate) / 12;
  newbal = bal +  total;
  return newbal;
 }



};

int main(){
 SavingsAccount saver1,saver2;
 double s1, s2;
 SavingsAccount::ModifyInterestRate(0.03);
 s1 = saver1.calculateMonthlyInterest(2000.00);
 s2 = saver2.calculateMonthlyInterest(3000.00);
 cout << "Monthly Interest Saver 1: " << s1 << endl;
 cout << "Monthly Interest Saver 2: " << s2 << endl;
 SavingsAccount::ModifyInterestRate(0.04);
 s1 = saver1.calculateMonthlyInterest(2000.00);
 s2 = saver2.calculateMonthlyInterest(3000.00);
 cout << "Monthly Interest Saver 1: " << s1 << endl;
 cout << "Monthly Interest Saver 2: " << s2 << endl;
 return 0;
}

The problem is on the static double AnnualInterestRate member. I have tried to fix this for 3 hours but still cannot please help my eyes hurt.

Errors:

Error LNK2001 unresolved external symbol "private: static double SavingsAccount::AnnualInterestRate" (?AnnualInterestRate@SavingsAccount@@0NA)
Error LNK1120 1 unresolved externals