我想知道如果用户想回到上一个问题，系统如何识别问题是否已经回答。如果答案已经被回答，答案将会更新，或者分数增加或者不被编辑，答案将会是相同的。
按钮1是返回到上一个问题按钮2是检查是真是假回答按钮3是继续考试

以下是我的代码片段：

Imports MySql.Data.MySqlClient
Imports System.Drawing
Imports System.IO
Public Class Exam
    'declaring variables for connection'
    Dim score As Integer
    Dim rightans As String
    Dim correct As Integer = 0
    Dim choice As String
    Dim choice1 As String
    Dim choice2 As String
    Dim choice3 As String
    Dim con As MySqlConnection
    Dim con1 As MySqlConnection
    Dim COMMAND As MySqlCommand
    Dim read As MySqlDataReader
    Dim da As MySqlDataAdapter
    Dim sql As String
    Private Sub Exam_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        'setting the radiobutton to false, so that when the form load there is no shaded button'
        Label2.Text = 1
        A.Checked = False
        B.Checked = False
        C.Checked = False
        D.Checked = False
        con = New MySqlConnection
        Button1.Enabled = False
        con.ConnectionString = "server=localhost;userid=root;password=;database=user;"
        'calling sub
        qno()
    End Sub
Sub clear()
        'to set the radiobutton false/no shaded.
        A.Checked = False
        B.Checked = False
        C.Checked = False
        D.Checked = False
    End Sub
    Sub qno()
        'calling connection'
        Try
            con = New MySqlConnection
            con.ConnectionString = "server=localhost; user id=root; password=; database=user;"
            con.Open()
            sql = "SELECT * FROM user.math WHERE question_id = @ID;"
            COMMAND = New MySqlCommand
            With COMMAND
                .Connection = con
                .CommandText = sql
                .Parameters.Clear()
                .Parameters.AddWithValue("@ID", Label2.Text)
                .ExecuteNonQuery()
            End With
            Dim arrImage() As Byte
            Dim dt As New DataTable
            da = New MySqlDataAdapter
            da.SelectCommand = COMMAND
            da.Fill(dt)
            If dt.Rows.Count > 0 Then
                arrImage = dt.Rows(0).Item(7)
                Dim mstream As New System.IO.MemoryStream(arrImage)
                Pic1.Image = Image.FromStream(mstream)
                question.Text = dt.Rows(0).Item(1)
                A.Text = dt.Rows(0).Item(2)
                B.Text = dt.Rows(0).Item(3)
                C.Text = dt.Rows(0).Item(4)
                D.Text = dt.Rows(0).Item(5)
            Else
                MsgBox("No results!")
            End If
        Catch ex As MySqlException
            MsgBox(ex.Message)
        Finally
            con.Close()
            da.Dispose()
        End Try
    End Sub
    Sub increment()
        'incrementing the score f the answer is correct'
        Dim i As Integer = 0
        i = Label2.Text
        i = i + 1
        Label2.Text = i
        If Label2.Text > 1 Then
            Button1.Enabled = True
        End If
    End Sub
    Sub decrement()
        'incrementing the score f the answer is correct'
        Dim i As Integer = 1
        i = Label2.Text
        i = i - 1
        Label2.Text = i
        If Label2.Text = 1 Then
            Button1.Enabled = False
        End If
    End Sub
    Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
        'checking of the user answer the questions'
        If ((A.Checked = False) And (B.Checked = False) And (C.Checked = False) And (D.Checked = False)) Then
            MsgBox("Please answer the question")
        Else
            'if the examinee answers all the examination it will call another questions from database'
            If A.Checked = True Then
                con.Open()
                ' Dim ans As String
                Dim arren As String = "A"
                Dim sql As String = ("select answer from user.math where question_id = '" & Label2.Text & "' ")
                COMMAND = New MySqlCommand(sql, con)
                Dim it As String
                read = COMMAND.ExecuteReader
                If read.HasRows Then
                    If read.Read Then
                        it = read.Item("answer")
                        If it = choice Then
                            correct = correct + 1
                            Label4.Text = correct
                        ElseIf it <> choice And Label2.Text <= 1 Then
                            correct = correct - 1
                            Label4.Text = correct
                        End If
                    End If
                    clear()
                End If
                If Label2.Text = 10 Then
                    MessageBox.Show("proceed to other subject test")
                End If
                con.Close()
    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        decrement()
        qno()
    End Sub
End Class