错误"system"模棱两可？

我有一个简单的程序，它工作得很好，但是system("CLS");和system("pause");语句在它们下面有红色的智能感知行。当我把光标移到它们上面时显示的是Error "system" is ambiguous.是什么原因?

#include <iostream>
#include <cmath>
using namespace std;
int main()
{
  int choice = 0;
  const double PI = 3.14159;
  double sideSquare = 0.0;
  double radius = 0.0;
  double base = 0.0;
  double height = 0.0;
  cout << "This program calculates areas of 3 different objects." << endl;
  cout << "1.) Square" << endl;
  cout << "2.) Circle" << endl;
  cout << "3.) Right Triangle" << endl;
  cout << "4.) Terminate Program" << endl << endl;
  cout << "Please [Enter] your object of choice: ";
  cin >> choice;
  system("CLS"); // The problem is here...
  switch(choice)
  {
   case 1: 
    cout << "Please [Enter] the length of the side of the square: ";
    cin >> sideSquare;
    cout << "The area is: " << pow(sideSquare, 2) << endl;
    break;
   case 2: 
    cout << "Please [Enter] the radius of the circle: ";
    cin >> radius;
    cout << "The area is: " << PI * pow(radius, 2) << endl;
    break;
    case 3:
    cout << "Please [Enter] the base of the triangle: ";
    cin >> base;
    cout << endl << "Now [Enter] the height of the triangle: ";
    cin >> height;
    cout << "The area is: " << (base * height) / 2 << endl;
    break;
  default:
    cout << "Please [Enter] a valid selection next time." << endl;
    return 0;
  }
  system("pause"); // ... and here.
  return 0;
}