9. Base your answer to question 9 on the passage below and on your knowledge of WWI:
… A weary, exhausted, nerve-racked group of men it was indeed that, about noon November 1, assembled in a gully north of Sommerance [France] to rest and dig in for the night. The artillery was still firing furiously, but the enemy’s barrage [bombardment] had ceased very suddenly about 10:00 a.m. and now only occasional shells from long-range rifles would explode in the vicinity. The weather was gloomy and the moist air chilled one to the bones. Yet it was with that meticulous care that is characteristic of worn-out men, that was prepared our foxholes, carrying boards and iron sheeting from abandoned machine-gunner’ dugouts as possible, evn though only for one night…
Source: William L. Langer, Gas and Flame in World War I, Knopf/Borzoi
Which means of warfare is described in this passage?