Final answer:
The two lines in the excerpt that show the painful and filthy conditions faced by soldiers in the trenches during World War I are (A) Obscene as cancer, bitter as the cud and (B) Of vile, incurable sores on innocent tongues.
Step-by-step explanation:
The two lines in the excerpt (A) Obscene as cancer, bitter as the cud and (B) Of vile, incurable sores on innocent tongues, show the painful and filthy conditions faced by soldiers in the trenches during World War I. These lines use vivid and graphic language to describe the suffering and physical ailments experienced by the soldiers, emphasizing the harsh and brutal reality of war.