สอบถามเรื่องโปรแกรมตัดเกรดอัติโนมัิติค่ะ(ผิดบอร์ดขออภัยนะคะ)

Public Class Form1
    Dim M, F, A, T As Integer
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
       

    End Sub

    Private Sub TextBox1_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox1.TextChanged

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        M = TextBox1.Text
        F = TextBox2.Text
        A = TextBox3.Text
        T = M + F + A
        TextBox4.Text = T
        Select T
            Case Is >= 80
                TextBox5.Text = ("A")
            Case Is >= 70
                TextBox5.Text = ("B")
            Case Is >= 60
                TextBox5.Text = ("C")
            Case Is >= 50
                TextBox5.Text = ("D")
            Case Else
                TextBox5.Text = ("F")
        End Select

        If T >= 80 Then
            TextBox5.ForeColor = Color.Green
        ElseIf T >= 70 Then
            TextBox5.ForeColor = Color.Green
        ElseIf T >= 60 Then
            TextBox5.ForeColor = Color.Green
        ElseIf T >= 50 Then
            TextBox5.ForeColor = Color.Green
        Else
            TextBox5.ForeColor = Color.Red

        End If
        If T >= 80 Then
            TextBox6.Text = "โห่!นายนี่เมพมากๆ"
        ElseIf T >= 70 Then
            TextBox6.Text = "เก่งฝุดๆเลยพี่น้องครับ"
        ElseIf T >= 60 Then
            TextBox6.Text = "C ก็จ๊าบแล้วพี่น้องครับ"
        ElseIf T >= 50 Then
            TextBox6.Text = "D นะที่ผ่านมาได้"
        Else
            TextBox6.Text = "คุณคือจุดอ่อน 555!"
        End If
    End Sub

    Private Sub TextBox5_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TextBox5.TextChanged

    End Sub
End Class