我的代码抛出断言错误有什么问题？

Question

这是我的代码，

using System;
using static System.Console;
using System.Globalization;
class ConvertMilesToKilometers
{
    static void Main()
    {
        // Write your code here
        double mi;
        double km;
        mi = GetMi();
        km = ConvertToKilometers(mi);
        WriteLine("{0} mile(s) is {1} km.",
            mi, km);
    }
    private static double GetMi()
    {
        double mi;
        Write("Enter the number of miles you would like to convert to kilometers >> ");
        mi = Convert.ToDouble(ReadLine());
        return mi;
    }

    private static double ConvertToKilometers(double mi)
    {
        double km;
        double CONVERSION_FACTOR = 1.60934;
        km = mi * CONVERSION_FACTOR;
        return km;
    } 
}   

这是 cengage 给出的错误： （据说是在测试该方法），

状态：失败！
检查：1
测试：用 10 英里测试`ConvertToKilometers`
原因：无法运行测试。
错误：str - 断言错误
时间戳: 2024-02-17 02:48:55.100907

状态：失败！
检查：2
测试：用 0 英里测试`ConvertToKilometers`
原因：无法运行测试。
错误：str - 断言错误
时间戳: 2024-02-17 02:49:04.118086

我输入了相同的数字，它对我来说效果很好，所以我不明白发生了什么

Answer 1

它最终成为访问修饰符。显然，Cengage 需要将其公开。这是最终的代码

using System;
using static System.Console;
using System.Globalization;
class ConvertMilesToKilometers
{
    static void Main()
    {
        // Write your code here
        double miles;
        double km;
        miles = GetMi();
        km = ConvertToKilometers(miles);
        WriteLine("{0} mile(s) is {1} km.",
            miles, km);
    }
    public static double GetMi()
    {
        double miles;
        Write("Enter the number of miles you would like to convert to kilometers >> ");
        miles = Convert.ToDouble(ReadLine());
        return miles;
    }

    public static double ConvertToKilometers(double miles)
    {
        double km;
        double CONVERSION_FACTOR = 1.60934;
        km = miles * CONVERSION_FACTOR;
        return km;
    } 
}