How Did Stalin Come to Succeed Lenin as Leader of the Soviet

How did Stalin come to succeed Lenin as leader of the Soviet Union?

A) Throughout the Revolution and the Civil War, Stalin was consistently Lenin's most high-profile comrade.
B) Though he was not a visible leader, Stalin used the position of General Secretary of the Communist Party to build up an unbeatable following that he adroitly used.
C) Due to his charisma, Stalin had built up a following that, though occasionally at odds with Lenin, formed a majority in the Party at Lenin's death.
D) Due to Lenin's failing health, Stalin officially shared authority with Lenin since the failed assassination attempt on Lenin in 1918.
E) Upon Lenin's death, the Party decided on a major change in direction, a change that surprised even Stalin.