Question about Estate and gift taxes: USA

Upon the death of Margret, all her assets are left to her younger sister, Jackie. The estate tax liability at Margret's death is $600,000 Jackie dies after Margret. The portion of Jackie's estate tax attributable to the assets Margret left to her is $700,000. The section 2013 credit available in Jackie's estate for the tax on prior transfers is:

A- $600,000 if Jackie died one year after Margret

B- $700,000 if Jackie died one year after Margret

C- $560,000 if Jackie died three years after Margret

D- $420,000 if Jackie died five years after Margret

E- $240,000 if Jackie died nine years after Margret