执行如下VB程序段时出现下图所示的错误，则引发此错误的语句不可能是（   ）

Private Sub Command1_Click()

Dim a As Integer, b As Integer

a =5

b = Val(Text1.Text)

b = a + b

label1.Caption = Str(b)

End Sub

A：Private Sub Command1_Click()

B：b = Val(Text1.Text)                         

C：b = a + b

D：label1.Caption = Str(b)