我对于yearRate，monthlyCharge和lateFee不断收到此错误。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Lab_5___Danny_Curro
{
    class Program
    {
        static void Main(string[] args)
        {
            string firstName;
            string lastName;
            int accNumber;
            string creditPlan;
            double balance;
            string status;
            Boolean late = false;
            double lateFee;
            double monthlyCharge;
            double annualRate;
            double netBalance;


            Console.Write("Enter First Name: ");
            firstName = Console.ReadLine();

            Console.Write("Enter Last Name: ");
            lastName = Console.ReadLine();

            Console.Write("Enter Account Number: ");
            accNumber = Convert.ToInt32(Console.ReadLine());


            Console.Write("Enter Credit Card Plan Number[Blank Will Enter Plan 0]: ");
            creditPlan = Console.ReadLine();

            Console.Write("Enter Balance: ");
            balance = Convert.ToDouble(Console.ReadLine());

            Console.Write("Is This Account Late?: ");
            status = Console.ReadLine().Trim().ToLower();

            if (creditPlan == "0")
            {
                annualRate = 0.35;  //35%
                lateFee = 0.0;
                monthlyCharge = balance * (annualRate * (1 / 12));
                return;
            }

            if (creditPlan == "1")
            {
                annualRate = 0.30;  //30%
                if (status == "y")
                {
                    late = true;
                }

                else if (status == "n")
                {
                    late = false;
                }
                if (late == true)
                {
                    lateFee = 25.00;
                }
                monthlyCharge = balance * (annualRate * (1 / 12));
                return;
            }
            if (creditPlan == "2")
            {
                annualRate = 0.20;  //20%
                if (status == "y")
                {
                    late = true;
                }

                else if (status == "n")
                {
                    late = false;
                }
                if (late == true)
                {
                    lateFee = 35.00;
                }
                if (balance > 100)
                {
                    monthlyCharge = balance * (annualRate * (1 / 12));
                }
                else
                {
                    monthlyCharge = 0;
                }
                return;
            }
            if (creditPlan == "3")
            {
                annualRate = 0.15;  //15%
                lateFee = 0.00;

                if (balance > 500)
                {
                    monthlyCharge = (balance - 500) * (annualRate * (1 / 12));
                }
                else
                {
                    monthlyCharge = 0;
                }
                return;
            }
            netBalance = balance - (lateFee + monthlyCharge);


            Console.WriteLine("Name: \t\t\t {0}  {1}", firstName, lastName);
            Console.WriteLine("Account Number: \t{0}", accNumber);
            Console.WriteLine("Credit Plane: \t\t{0}",creditPlan);
            Console.WriteLine("Account Late: \t\t{0}", late);
            Console.WriteLine("Balance: \t\t{0}", balance);
            Console.WriteLine("Late Fee: \t\t{0}", lateFee);
            Console.WriteLine("Interest Charge: \t{0}", monthlyCharge);
            Console.WriteLine("Net Balance: \t\t{0}",netBalance);
            Console.WriteLine("Annual Rate: \t\t{0}", annualRate);
            Console.ReadKey();
        }
    }
}