执行如下Visual Basic程序段时，出现了如下图所示的错误：

Private Sub Command1_Click()
Dim a As Integer, b As Integer
a =" 5:" b = 8
b =" a" + b
label1.Caption = Str(b)
End Sub
引发此错误的语句可能为（ ）

A．Dim a As Integer, b As Integer	B．a =" 5:" b = 8
C．b =" a" + b	D．label1.Caption = Str(b)