A US Army M1A2 SEP tank participates in the Strong Europe Tank Challenge 2017 at the military training area in Grafenwoehr, Germany, 12 May 2017. Six North Atlantic Treaty Organization (NATO) nations and partner nations, Austria, France, Germany, Poland, Ukraine and the USA are taking part in this competing exercise. (Photo by Christian Bruna/EPA)

Military officers look at an AR-10 sniper rifle at LAAD, the biggest military industry expo in Latin America in Sao Paulo, Brazil April 10, 2018. (Photo by Nacho Doce/Reuters)

Competitors take part in the annual Mud Madness event at Foymore Lodge on April 22, 2018 in Portadown, Northern Ireland. The adult version of the event includes two laps of an 8km course through 25 obstacles while the kids event is run over 2kms. The race is in it's eleventh year and is sponsored by McVities Jaffa Cakes and event charity partner Marie Curie together with other numerous charities and fundraising groups. (Photo by Charles McQuillan/Getty Images)

Aerial photo taken on April 20, 2018 shows the view of advection fog above Qingdao, a coastal city in east China' s Shandong Province. (Photo by Lu Hui/Xinhua News Agency/Eyevine)

Cambridge students the morning after the Trinity and Jesus college May Balls in Cambridge, United Kingdom on June 19, 2018. Students at Cambridge University celebrate the end of the academic year with the May Balls. (Photo by South West News Service)

Participants of the “Torture Ships” walk to the pier where the Ship of lacquer and leather will undock with hundreds watching in Friedrichshafen, Germany on June 23, 2018. (Photo by Felix Kästle/DPA)

A chicken gingerly crosses the double yellow line of a country road near Roseburg in western Oregon, USA on July 16, 2018. (Photo by Robin Loznak/ZUMA Wire/Alamy Live News)

A Russian police officer detains a teenager during rally protesting retirement age hikes in St. Petersburg, Russia, Sunday, September 9, 2018. A government plan to increase the age for collecting state pensions brought protests across Russia's 11 time zones on Sunday even though the opposition leader who called them was in jail. Nearly 300 people were reported arrested. (Photo by Roman Pimenov/Interpress photo via AP Photo)