You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Alexander_Graham_Bell

Context:
Meanwhile, Elisha Gray was also experimenting with acoustic telegraphy and thought of a way to transmit speech using a water transmitter. On February 14, 1876, Gray filed a caveat with the U.S. Patent Office for a telephone design that used a water transmitter. That same morning, Bell's lawyer filed Bell's application with the patent office. There is considerable debate about who arrived first and Gray later challenged the primacy of Bell's patent. Bell was in Boston on February 14 and did not arrive in Washington until February 26.

Question:
What office in Washington did Bell and Gray visit?

U.S. Patent Office