Cannot Close Excel

[huyhk]

My code is

Public xApp As Excel.Application
Private Sub Command1_Click()
    Dim wb As New Excel.Workbook, str As String
    Set xApp = CreateObject("Excel.Application")    
    xApp.Visible = True
    str = "Myfile.xls"
    Set wb = xApp.Workbooks.Open(FileName:=str)
    
    wb.Close False
End Sub

Private Sub Command2_Click()
    xApp.Quit
    Set xApp = Nothing
End Sub

When I click on Command2, Excel window is closed but an Excel process always appears in Window Task Manager,Process Tab. If I run above code again, two Excel processes are in Window Task Manager and so on.
Anyone can help me to terminate Excel processes?