Answer the question from the given passage. Your answer should be directly extracted from the passage, and it should be a single entity, name, or number, not a sentence.

Let me give you an example: Passage: Martin Luther married Katharina von Bora, one of 12 nuns he had helped escape from the Nimbschen Cistercian convent in April 1523, when he arranged for them to be smuggled out in herring barrels. Suddenly, and while I was occupied with far different thoughts, he wrote to Wenceslaus Link, “the Lord has plunged me into marriage. At the time of their marriage, Katharina was 26 years old and Luther was 41 years old. Question: In a letter who did Luther credit for his union with Katharina?
The answer to this example can be: the Lord
Here is why: The paragraph clearly states that, Luther has credited the Lord for his union with Katharina, hence the Lord is correct answer.

OK. solve this:
Passage: Southern California is home to Los Angeles International Airport, the second-busiest airport in the United States by passenger volume (see World's busiest airports by passenger traffic) and the third by international passenger volume (see Busiest airports in the United States by international passenger traffic); San Diego International Airport the busiest single runway airport in the world; Van Nuys Airport, the world's busiest general aviation airport; major commercial airports at Orange County, Bakersfield, Ontario, Burbank and Long Beach; and numerous smaller commercial and general aviation airports. Question: What is the metric they use to determine how busy airports are?
Answer:
passenger volume