设计VB程序，实现功能：将每个科目的选课人数按照选考人数降序排序后输出。其中，科目名称存放在数组a(i)中，人数存放在数组b(i)中。

程序运行界面设计如图所示，左边列表框List1是显示科目名称和选课人数，单击“排序”按钮Command1后，在右边的列表框List2中显示排序后的科目名称和选课人数。

Command1的处理事件过程如下，但加框处代码有错，请改正。

Private Sub command1_click()

Dim a(1 To 7)  As String

Dim b(1 To 7)  As Integer

Dim i As Integer, j As Integer, p As String, q As Integer

For i = 1 To 7

   List1.AddItem a(i) + " " + Str(b(i))

Next i

For i = 1 To 6

   k = i

   For j = i + 1 To 7

Next j

   If i <> k Then

     p = a(i): a(i) = a(k): a(k) = p

   End If

Next i

For i = 1 To 7

   List2.AddItem a(i) + " " + Str(b(i))

Next i

End Sub
①{#blank#}1{#/blank#}   ②{#blank#}2{#/blank#}

n = Val(Text1.Text)

For i = 1 to n

 a(i) = i

Next i

For i = 1 to n/2-1

 For j = n to 2*(i+1) step -2

  If      ①    Then t = a(j):a(j)=a(j-2):a(j-2)=t

 Next j

For j = 1 to n- i*2 step 2

  If      ②    Then t = a(j):a(j)=a(j+2):a(j+2)=t

 Next j

Next i

Dim tmp As Integer

Dim a(1 To 10) As Integer

Private Sub Command1_Click()

  For i=2 To 10

Text1.Text = ""

For i= 1 To 5

  For j=i+1 To 6

  If s(i) + s(i) < s(j) + s(i) Then

    t= s(j): s(j)= s(i): s(i)=t

  End If

  Next j

Next i

For i=1 To 6

  Text1.Text = s(i) + Text1.Text

Next i

  If  Then

    tmp = a(i)

      For j=  To 1 Step -1    ‘依次后移

        If tmp > a(j) Then Exit For

        a(j+1)= a(j)

      Next j

          ‘插入数据

    End If

  Next i

  For i= 1 To 10

    List1.Addltem a(i)

  Next i

End Sub

要实现上述功能，方框①②③中的语句分别是（  ）

For i=1To n＼2   

    k=i

        If a(j) <a(k) Then k=j

    Next j

    If i<>k Then

        t=a(i)：a(i)=a(k)：a(k)=t

    End If

    If  Then

        t=a(j)：a(j)=a(j+1)：a(j+1)=t

      End If

    Next j

Next i

上述程序中方框处可选语句或表达式有：

①For j=i+1 To n-i+ 1    ②For j=i+1 To n-i

③a(j)<a(j+1) ④a(j)>a(j+1)

则(1)、(2)、(3)处语句依次是（  ）

max=0

For i=1 to 7

    k=i

    For j=i+1 to 8

         If a(k)<a(j) then k=j

    Next j

    If k<>i then

        t=a(k)：a(k)=a(i)：a(i)=t

        If max<k-i then max=k-i

    End If

Next i

数组元素a(1)到a(8)的值依次为：19，9，48，32，5，17，23，8，执行该程序段后max的值为(      )

p = 1: q = 10

Do While p < q

    iMin = p: iMax = p

    For i = p + 1 To q

        If a(i) < a(iMin) Then

            iMin = i

        elseIf a(i) > a(iMax) Then

            iMax = i

        endif

    Next i

    t = a(iMax): a(iMax) = a(q): a(q) = t

      ①  

    t = a(iMin): a(iMin) = a(p): a(p) = t

    p = p + 1:q = q -1

Loop

要使程序实现上述算法思想，则方框中的语句是（  ）