How Did Gaius Marius Transform the Roman Military

How did Gaius Marius transform the Roman military?

A) He developed a professional corps of engineers assigned to each legion to provide onsite advice and solutions to situations that arose.
B) By permitting slaves to serve in the army with possibilities for freedom for service, he expanded the size of military forces dramatically.
C) By permitting landless men to serve in the army, he tapped a large reservoir of idle manpower.
D) He established uniform military laws to which both officers and common soldiers were subject, which also gave them significant privileges for their service.