Sam's grandparents want to refinance their house. With his knowledge from finance class, Sam gathered the data as followed to help them: their $300,000 old mortgage was 5.76% APR, now the bank offers them 4.2% APR on a similar 15 year-mortgage. How much is the saving on their monthly mortgage payment from refinancing? :

A) $243.59

B) $219.86

C) $346.51

D) $113