B. Register for the draft.
Step-by-step explanation:
The Selective Service Act was ratified on May 18, 1917. This made it so men ages 21 through 30 were registered for the war draft. This later got changed to men aged 18 to 45.
During this time, the United States was just entering World War I. Not a lot of Americans were joining to fight in the war, which proved to be a problem. Even after making a big deal about this, still not a lot of people joined. A solution needed to be created.
This was when the Selective Service Act was created, as the United States needed soldiers to fight in World War I.