which subsidiary of air china serves both guangzhou baiyun international airport and nanjing lukou international airport ?
select distinct var_uri where brack_open dbr_Air_China dbo_subsidiary var_uri sep_dot var_uri dbo_targetAirport dbr_Guangzhou_Baiyun_International_Airport sep_dot var_uri dbo_targetAirport dbr_Nanjing_Lukou_International_Airport sep_dot brack_close