Obtaining an investment-grade rating will keep the county's future borrowing costs low, protect its already-tattered image, and increase its ability to buy bond insurance. (A) Obtaining an investment-grade rating will keep the county's future borrowing costs low, protect (B) To obtain an investment-grade rating will keep the county's future borrowing costs low, and protect (C) Having obtained an investment-grade rating will, in keeping the county's future borrowing costs low, protect (D) To obtain an investment-grade rating would keep the county's future borrowing costs low, protecting (E) Obtaining an investment-grade rating, keeping the county's borrowing costs low, would be protecting to obtain和obtaining有什么区别?D错在哪儿,protecting做插入语,keep和increase平行不行吗?这句话主语是Obtaining an investment-grade rating?