The BEST explanation for why approximately 3,000,000 American women joined the workforce in 1918 is:
B They joined the workforce to offset the labor shortage caused by America's involvement in World War I.
During World War I, many men left their jobs to serve in the military, creating a significant labor shortage. To fill these positions and support the war effort, a large number of women entered the workforce, taking on roles in factories, offices, and other industries.