Private Sub Command1_Click()

Dim a(1 To 4) As Integer

Dim i As Integer

   a(1) = 1

 For i = 2 To 4

   a(i) = 2 * a(i - 1) + 1

 Next i

List1.AddItem Str(a(i))

End Sub

图1

Private   Sub  Command1_click ( )

Dim  n  As  Integer , x  As  Integer

n = Val ( Text1．Text  )

x = n  Mod 10

n = x * 1000 + n \ 10

Text1．Text = Str ( n )

End  Sub

Private Sub Command1_Click()

  Dim a As Integer, b As Integer

  a = 10                                        '①

  b = 20                                        '②

  Text1.Text = "a+b=" + Val(a + b)              '③

End Sub                                         '④

Private Sub Command1_Click()

Dim r As Single, s As Single            '①

r="10米"                     '②

s=3.14159*r*r            '③

Text2.Text ="某个半径为"+ Str(r)+"米的圆,其面积为"+ Str(s)+"平方米."             '④

End Sub

则程序中出错的语句是（   ）