Passengers wait at the departure hall of the Beijing Railway Station in central Beijing, China January 27, 2017 as China gears up for Lunar New Year, when hundreds of millions of people head home. (Photo by Damir Sagolj/Reuters)

Laborers lay down upon a field to cut off the tulips that remained uncut by the tractor in Den Helder, Netherlands April 22, 2017. (Photo by Cris Toala Olivares/Reuters)

National Guardsmen are put through riot training in Boston's Commonwealth Armory on Friday, October 18, 1974. Massachusetts Gov. Francis W. Sargent called up the guard to quell school violence, but the city has been relatively calm and the guard has remained in the armories. (Photo by JWG/AP Photo)

A reporter takes pictures of a soldier during the annual Han Kuang military exercise in Kinmen, Taiwan, September 7, 2015. Rustic Kinmen, with a population of less than 129,000, is a half-hour ferry ride to China, but it takes an hour to fly to major Taiwan cities. Just off its shores, glass-walled high-rises wink seductively from the booming mainland port of Xiamen in one of China's most prosperous provinces. (Photo by Pichi Chuang/Reuters)

An Iraqi Muslim woman walks past Christmas decorations for sale in the Shiite holy city of Najaf on December 17, 2015. (Photo by Haidar Hamdani/AFP Photo)

A view of a stampede is seen during a New Year's celebration on the Bund, a waterfront area in central Shanghai, December 31, 2014. The stampede killed at least 35 people and injured 43 during New Year's Eve celebrations in Shanghai, on the city's famed waterfront tourist strip known as the Bund, authorities said. (Photo by Reuters/Stringer)

Soccer player David Beckham (C) with Cruz Beckham (L) and Romeo Beckham (R) get slimed onstage during Nickelodeon Kids' Choice Sports Awards 2014 at UCLA's Pauley Pavilion on July 17, 2014 in Los Angeles, California. (Photo by Kevin Winter/Getty Images)

People look at home-made armoured vehicle look-alikes on a street in Shenyang, Liaoning province, November 12, 2014. A man surnamed Zhang and his friends converted two cars into these two vehicles, for his son, and will be displaying them on show at a local park. The cannons on the vehicles can fire paintballs and smoke shells, local media cited Zhang as saying. (Photo by Reuters/Stringer)