The Rough Riders were a volunteer cavalry military unit from the US, raised in 1898 to participate in combats during the Spanish-American War. The US had decided to intervene in the conflict between Cuba and Spain after the explosion of the cruise USS Maine, for which the Spanish were blamed.
President McKingley had called for volunteers due to the situation of the army, which was small and understaffed, if compared to previous conflicts. <u>Being constituted by volunteers</u> fostered the sucess of this military unit as they were fueled by an inner passion towards defending their country and its prestige, therefore they fought with this extra motivation.
I believe that A and B are correct, and if you need 3 than A, B, and D are correct.
