The special shells the men call shells on wheels" are whizzing by continuously. they explode silently and have no smell but can be deadly. they killed several men yesterday. one of my men refused to put his mask on because he couldn't smell anything. all of a sudden, he was dizzy, foaming at the mouth and his skin went black, then he went rigid and died." —paul truffaut, march 5, 1917 in this text, which form of warfare from world war i is truffaut discussing?