challenge: What event in the early 2010s led to a significant increase in air activity by China near Okinawa?
  
  A. The United States' deployment of a new aircraft carrier to the region
  B. Japan's decision to establish a new military base on Okinawa
  C. Japan's effective nationalization of the disputed Senkaku Islands
  D. China's establishment of an air defense identification zone over the East China Sea
  Answer: 