Sally has three brothers._____ is a doctor,_____ are drivers.

答案是B
the other指两个人或物中的一个时,只能用the other,不能用another,此时的other作代词.如：
He has two daughters.One is a nurse,the other is a worker.
他有两个女儿,一个是护士,另一个是工人.
the other后可加单数名词,也可加复数名词,此时的other作形容词.如：
On the other side of the street,there is a tall tree.
在街道的另一边,有一棵大树.
Mary is much taller than the other girls.
玛丽比其他的女孩高得多.
He lives on the other side of the river.
他住在河的对岸.
the others意思是“其他东西,其余的人”.特指某一范围内的“其他的（人或物）”.是the other的复数形式.如：
Two boys will go to the zoo,and the others will stay at home.
两个男孩将去动物园,其余的留在家里.
the others=the other +复数名词