C＃不知道从这里去哪里

我已经从事此功课已有一段时间了，我还有2个问题要解决……这个问题特别困扰我，我什至不确定我明白它的要求了。我朝着正确的方向...

**对于列表letterGrades中的所有元素如果索引i处的元素等于等级，则将noOfGrades递增1。无论大小写，都应进行此比较，例如p / P / f / F。

基本前端程序

这是我到目前为止拥有的当前代码清单...

/*
 * Calculate letter grades as P for Pass and F for Fail based on the marks obtained.
 * Find the number of instances with the given grades.
 */
namespace Ex1
{
    public partial class LetterGrade : Form
    {
        // Initialize string List
        public List<string> letterGrades = new List<string>();

        public LetterGrade()
        {
            InitializeComponent();
        }


        private void submitButton_Click(object sender, EventArgs e)
        {
            double marks = double.Parse(marksTextbox.Text); //Declare variable marks and set

            // If statements for marks
            if (marks>0 && marks<=60)
            {
                letterGrades.Add("F");
            }
            else if (marks>60 && marks<=100)
            {
                letterGrades.Add("P");
            }
            else
            {
                MessageBox.Show("Not a valid marks");
            }
            marksTextbox.Clear();
        }


        private void calculateButton_Click(object sender, EventArgs e)
        {
            string grade = letterGradeTextbox.Text; // initialize string grade
            int noOfGrades = 0;

            for (int i = 0; i < letterGrades.Count; i++)
            {
                if (grade == "P" || grade == "p") //Problem here, step 7a...could not initialize using [i]..tried using a workaround but it keeps total count..
                {
                    noOfGrades++;
                }
                else if (grade == "F" || grade == "f") 
                {
                    noOfGrades++;
                }
            }

            letterGradeTextbox.Clear();

            MessageBox.Show("Number of instances with given letter grade is: " + noOfGrades);
        }
    }
}

我最有可能在月球和月球上想这件事，但是如果有人可以将我指向右边的兔子洞，我将非常感激