Dim j  As Integer, k  As Integer

List Clear

  For k = 1 To 100

     j = Int(Sqr(k))

     If j * j = k Then list1.AddItem Str(k)

 Next k

    分析程序功能，该程序的功能是（   ）

当输入的运算符不是加、减、乘、除四种运算符时，则输出“运算符不正确”；当进行除法运算时，如果除数为0，提示输出“除数不能为零”。相应的Visual Basic程序如下，在①和②划线处，填入合适的语句或表达式，把程序补充完整。

Private Sub Command1_Click（）

    Dim a As Single, b As Single, ch As String

    a = Val(Text1.Text)

    b = Val(Text2.Text)

             ①      

    If ch = "+" Then

        Text4.Text = Str(a + b)

    ElseIf ch = "*" Then

        Text4.Text = Str(a * b)

    ElseIf ch = "-" Then

        Text4.Text = Str(a - b)

    ElseIf ch = "/" Then

        If  ②  Then Text4.Text = Str(a / b) Else Text4.Text = "除数不能是零"

    Else

                 ③     

    End If

End Sub

Private Sub Command1_C1ick ( )

Dim a(9) As Integer

a(9)=Val(Text1.text)

For i=8 to 1 step -2

a(i)=a(i+1)/2

Next i

Text2.text=str(a(i))

End Sub

Dim s As Double, x As Double, n As Integer, i As Integer

s=0:n=0

For i= 1 to 4

  n=n+1

  x=(n-1)/(n+1)

  s=s+x

Next i

该程序段计算的数学表达式是（   ）

i = 1

 Do While i <= 6

    t = Int(Rnd * 10) + 1

    If t Mod 2 = i Mod 2 Then a(i) = t : i = i + 1

Loop

For i = 1 To 2

   k = 1

   For j = 1 To 6 - i * 2

     If a(j) * k > a(j + 2) * k Then

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

     End If

     k = -k

   Next j

Next i

执行该程序段后，数组元素a(1)到a(6)的值可能是（  ）