GMAT Sentence Correction (SC) | Re: Bill and Lissy believed that if they were coached everyday and dedicat

Bill and Lissy believed that if they were coached everyday and dedicated themselves to practice,their dream of becoming a professional skater could someday be a reality .

(A) No change
(B) their dream of becoming professional skaters could someday be a reality
(C) their dreams of becoming professional skaters could someday be a reality
(D) their dreams of becoming professional skaters could someday be realities

Please explain in detail why each choice is correct and incorrect.