MonoDevelop上的命名空间错误

我收到以下错误。1 ：类型或命名空间名称"CountDownTime"在命名空间"系统"中不存在（您是否缺少程序集引用）2：类型或命名空间naem'运行时'在命名空间Andriod.OS中不存在（你是否缺少程序集引用）

我希望代码生成 10 个减法问题，提出答案，然后给出在测试上花费的时间。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.CountDownTimer;
using Android.App;
using Android.Content;
using Android.OS.Runtime;
using Android.Runtime;
using Android.Util;
using Android.Views;
using Android.Widget;
using Android;
namespace Jagtutor
{
    public class Subtraction : View
    {
        public Subtraction (Context context, IAttributeSet attrs) :
            base (context, attrs)
        {
            Initialize ();
        }
        public Subtraction (Context context, IAttributeSet attrs, int defStyle) :
            base (context, attrs, defStyle)
        {
            Initialize ();
        }
        private void Initialize ()
        {
            int correctCount;
            int count = 0;
            long startTime = CountDownTimer(0);
            while (count < 10)
            {
                // Generate two random single-digit numbers
                srand(CountDownTimer(0));
                int number1 = Random() % 10;
                int number2 = Random() % 10;
                // if number1 < number, swap number1 with number2
                if (number1 < number2)
                {
                    int temp = number1;
                    number1 = number2;
                    number2 = temp; 
                    // PROMPT THE STUDENT TO ANSWER " WHAT IS NUMBERE1 - NUMBER2?"
                    Console.WriteLine("WHAT IS ")(number1);" - "(number2)("?");
                    // Grade the answer and display the result
                    if (number1 - number2 == answer){
                        Console.Write("You are correct!");
                        correctCount++;
                    }
                    else
                        Console.WriteLine("Your answer is wrong");
                    Console.WriteLine(number1);"-"(number2); " should be" (number1 - number2);
                    // increase the count 
                    count++;
                }
                long endTime = CountDownTimer(0);
                long testTime = endTime - startTime; 
                Console.Write(" Correct count is ")(correctCount);" Test time is" (testTime)("seconds");
                return 0;
            }
        }
    }
}

using Android.OS;