C# XML parser

Given an XML file, which can be a string, passed from another application (for now it is an xml file), need C# code to read the file and then find the data for 3 specific data points

1) Instrument_Ratings/Instrument_Rating/Rating_Text WHERE Rating_Termination_local_date is Blank
2) Instrument_Identifiers/Instrument_Identifier/Instrument_ID_Value WHERE ID_Type_Text = "CUSIP"
3) 2) Instrument_Identifiers/Instrument_Identifier/Instrument_ID_Value WHERE ID_Type_Text = "ISIN" (If Available)