visual C++.程序在输入后继续退出

我的代码有问题。所以我的作业是这样问的：

一家移动电话服务公司为其客户提供三种不同的订阅套餐：

套餐A：每月39.99美元，提供450分钟。额外的分钟数为每分钟0.45美元。

套餐B：每月59.99美元，提供900分钟服务。额外的分钟数为每分钟0.40美元。

套餐C：每月69.99美元，提供无限分钟。

编写一个计算客户每月账单的程序。它应该询问客户购买了哪个包裹以及使用了多少分钟。然后，它应该显示到期总金额。

输入验证：确保用户只选择包A、B或C。

这是我的代码：

/*
1. Set variables (chars, int, etc) for hours and fees.
2. Ask user to select between A, B, or C.
3. Ask user to input time.
Also set a maximum amount of time for each case and setting a maximum amount of time for the     month.
5.Use case switch for options
6.calculate the customers bill for the month apprioprately.
*/

#include <iostream> 
#include <iomanip> 
#include <cmath>
using namespace std;
int main()
{
const double packageA = 39.99;//set variables for all three. They don't change. 
const double packageB = 59.99;
const double packageC = 69.99;
char choices = ' '; // use "char" for the packages(choices)
int time = 0; // set time as integer. Since some may be decimals, I use double. 
double letter = 0.0; //use "letter" for whatever letter they choose. 
cout << "Read choices below and select choice."<<endl;
cout << "A.$39.99 per month gets 450 minutes. Additional minutes are $0.45 per minute." << endl;
cout << "B.$59.99 per month gets 900 minutes. Additional minutes are $0.40 per minute." << endl;
cout << "C.$69.99 per month gets you unlimited access" << endl;

cout << "Select A, B, or C" << endl;
cin >> letter;
if (choices == 'A' || choices == 'B' || choices == 'C')//using switch case
{
    cout << "Enter minutes:" << endl;//ask user to input time
    cin >> time;

    if (time>0 && time<43829)// 43829 is the max amount of minutes in a month. 0 is the least they person can have. If it fits the requirements, then it can continue. 
    {
        switch (choices)
        {
        case 'A':
            if (time<450)
                letter = packageA;// if the time is less than required. Then no extra charge. 
            else
                letter = ((time - 450)*0.45) + packageA;// if it exceeds maximum minutes and 45 cents is charged. Same for all cases below except its respective amount is charged. 
            break;
        case 'B':
            if (time<900)
                letter = packageB;
            else
                letter = ((time - 900)*.40) + packageB;
            break;
        case 'C':
            letter = packageC;// if not, then package C and no equation since time is unlimited. It is a one time fee for all time used. 
            break;

        default: cout << "Total amount due is: $" << letter << endl; // give total amount charged based on information entered. 
        }
    }
    system("pause");
    return 0;
}
}

我的问题是，当我运行它时，它会在我选择一封信后关闭。如果我选择A，它会自动关闭。我在它关闭后收到这条消息：

"ConsoleApplication7.exe"（Win32）：已加载"C:\Users\Prince\Documents\Visual Studio 2013\Projects\ConsoleApplication7\Debug\ConsoleApplication7.exe"。已加载符号。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Windows\SysWOW64\ntdll.dll"。找不到或打开PDB文件。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Windows\SysWOW64\kernel32.dll"。找不到或打开PDB文件。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Windows\SysWOW64\KernelBase.dll"。找不到或打开PDB文件。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Program Files\Bitdefender\Bitdefender 2015\active virus control\Avc3_00259_008\avcuf32.dll"。找不到或打开PDB文件。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Windows\SysWOW64\msvcp120d.dll"。找不到或打开PDB文件。

"ConsoleApplication7.exe"（Win32）：已加载"C:\Windows\SysWOW64\msvcr120d.dll"。找不到或打开PDB文件。

程序"[3476]ConsoleApplication7.exe"已退出，代码为0（0x0）。