Given a text T, and several options, according to the question posed in the text T to choose the most appropriate option from the options as the answer, please directly output the most appropriate option, do not need to output the explanation.

Which of the following is most likely the underlying of a plain vanilla interest rate swap?
Option A:180-day Libor
Option B:10-year US Treasury bond
Option C:Bloomberg Barclay’s US Aggregate Bond Index

Answer: A