某Visual Basic程序如下:
Private Sub Command1_Click()
Dim a As String, b As String
a = "up "
b ="look "+" "+a
Label1.Caption = b
End Sub
运行程序,单击命令按钮Command1后弹出如下图所示的出错信息提示,此时程序正执行的语句是( )

A.Dim a As String, b A s String | B.a = "up " |
C.b = "look "+" "+a | D.Label1.Caption = b |