Can you please explain this question?

8)A CMBS issuance is $1,550,000,000, of which the AAA tranches total $1,356,250,000 of principal balance, the AA tranche is $62,000,000 and the Single A tranche is $55,800,000.The subordination level, expressed as a percent, of the of the Single A ('A') tranche is

a)11%

b)4.9%

c)8.5%

d)3.9%