Private Sub Commandl_Click()

     Dim i As Integer,j As Integer.s As String

     Dim a(1 To 5) As Integer

     List1.Clear

     Randomize             

     For i=1 To 10

       j=Int(Rnd*6)+1

       a(j)=a(j)+1

Next i

For i=1 To 5

  s=Str(a(i))

  List1.AddItem s

Next i

End Sub

程序运行时，单击按钮Commandl后弹出如图所示的错误提示。出现错误的语句是(   )

Private Sub Command1_Click()

Dim sum As Double, x As Double

sum = 0

n = 0

For i = 1 To 5

x = n / i

n = n + 1

sum = sum + x

Next i

End Sub

该程序通过For循环计算一个表达式的值，这个表达式是（   ）

Private Sub Command1_Click()

    Dim a As Integer, b As Integer

    Dim c As Integer                     '①

    a= Val(Text1. Text)                     '②

    b = Val(Text2. Text)

    c=a + b                             '③

    Label1. Caption ="a + b="+c        '④

End Sub