Q: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Why is Philadelphia a tourist attraction?, Context: The area's many universities and colleges make Philadelphia a top international study destination, as the city has evolved into an educational and economic hub. With a gross domestic product of $388 billion, Philadelphia ranks ninth among world cities and fourth in the nation. Philadelphia is the center of economic activity in Pennsylvania and is home to seven Fortune 1000 companies. The Philadelphia skyline is growing, with several nationally prominent skyscrapers. The city is known for its arts, culture, and history, attracting over 39 million domestic tourists in 2013. Philadelphia has more outdoor sculptures and murals than any other American city, and Fairmount Park is the largest landscaped urban park in the world. The 67 National Historic Landmarks in the city helped account for the $10 billion generated by tourism. Philadelphia is the birthplace of the United States Marine Corps, and is also the home of many U.S. firsts, including the first library (1731), first hospital (1751) and medical school (1765), first Capitol (1777), first stock exchange (1790), first zoo (1874), and first business school (1881). Philadelphia is the only World Heritage City in the United States.
A:
67 National Historic Landmarks in the city