What Spurred Islam's Remarkably Rapid Territorial Expansion Under the "Rightly

What spurred Islam's remarkably rapid territorial expansion under the "rightly guided" caliphs?

A) The caliphs declined to convert nonbelievers and tolerated diverse beliefs, which permitted Islam to adapt to many different societies.
B) The caliphs moved the capital to Baghdad, which was at a nexus of trade routes that allowed rapid military movement.
C) The caliphs promoted the Five Pillars, which required the forced conversion on Christians and Jews.
D) The caliphs linked religious uprightness with territorial expansion and empire building.