instruction: How many people visit the zoo in the US each year?
context: A zoo (short for zoological garden; also called an animal park or menagerie) is a facility in which animals are kept within enclosures for public exhibition and often bred for conservation purposes.
  
  The term zoological garden refers to zoology, the study of animals. The term is derived from the Greek ζώον, zoon, 'animal', and the suffix -λογία, -logia, 'study of'. The abbreviation zoo was first used of the London Zoological Gardens, which was opened for scientific study in 1828 and to the public in 1847. In the United States alone, zoos are visited by over 181 million people annually.
response: 181 million people in the US visit the zoo annually each year.