You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

[Q]: Category: FAMOUS FIRSTS
Clue: In 1876 this man became the first person not to get a busy signal on the other end of a phone line
[A]: alexander graham bell

[Q]: Category: THE ARMED SERVICES
Clue: In September 1997 this branch of the U.S. Armed Services celebrated its 50th anniversary
[A]: u.s. air force

[Q]: Category: EXPLORERS
Clue: He joined the London Missionary Society in 1838; after graduating medical school in 1840 he hoped to be sent to China
[A]:
david livingstone