I THINK ITS D OR B? NOT SURE, CORRECT ME IF WRONG PLEASE, TIA!

Progressive Era reformers after 1900 sought federal laws to regulate working conditions and to set a minimum age for child workers. The need for these laws was prompted by which of the following?

A) unions restricting membership to adults only?

B) civil war casualties leading to children taking the jobs of adults?

C) lack of job opportunities for children seeking after school jobs?

D) industrialists using child labor to keep production costs low?