[单项选择]Campaigning on the Indian frontier is an experience by itself. Neither the landscape nor the people find their counterparts in any other portion of the globe. Valley walls rise steeply five or six thousand feet on every side. The columns crawl through a maze of giant corridors down which fierce snow-fed torrents foam under skies of brass. Amid these scenes of savage brilliancy there dwells a race whose qualities seem to harmonize with their environment. Except at harvest-time, when self-preservation requires a temporary truce, the Pathan tribes are always engaged in private or public war. Every man is a warrior, a politician and a theologian. Every large house is a real feudal fortress made, it is true, only of sun-baked clay, but with battlements, turrets, loopholes, drawbridges, etc. complete. Every village has its defence. Every family cultivates its vendetta; every clan, its feud. The numerous tribes and combinations of tribes all have their accounts to settle with one another. Not
A. the introduction of the rifle
B. the Spread of British rule
C. the extension of luxuries
D. the spread of trade