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_Click()

Dim i As Integer, c As Integer, n As Integer

n = Val(Text1.Text)

c = 0

For i = 1 To n

If i Mod 3 = 0 Then c = c + 1

Next i

Label1.Caption = Str(c)

End Sub

运行该程序，在文本框Text1中输入20，单击命令按钮Command1，在标签Label1中显示的是{#blank#}1{#/blank#}。