Answer:
C. Chemical weapons.
Explanation:
TL:DR Chemical gas is a war crime due to the amount of suffering it causes. Mustard gas burns the skin, Phosgene is impossible to see, and Chlorine can make you drown in your own body.
Machine guns had been in use and invented since 1908, with invention of weapons such as the Vickers and German Maschinegewehr 08. Trench warfare wasn't a weapon, but more of a tactic. Mines weren't a war crime either as they most likely weren't deemed to cause "unneeded suffering" and their purpose usually was to simply discourage the travel of an area. Usually, if you stepped on a mine you'd die.
Meanwhile, Chemical gas was simply invented on accident as the German Empire's scientist,
<span>the invention of elevators.</span>
I believe it was France in the D-Day Invasions, that was a massive assault on the beaches and ended in the Allied forces pushing through the Nazi defenses.
The U.S politicians and businessmen worried about the foreign nations controlled economic development so they supported open door policy
