Which firm would be most likely to have an investment grade credit rating, all else being equal?

Select an answer:

A$.$a firm with capital sctructure of $30\%$ equity, $70\%$ debt

B$.$a firm with capital sctructure of $10\%$ equity, $90\%$ debt

C$.$a firm with capital sctructure of $80\%$ equity, $20\%$ debt

D$.$a firm with capital sctructure of $50\%$ equity, $50\%$ debt