Real Estate Law: Deeds and Covenants

CASE:
Aaron owns a farm.  Aaron has given to First Bank and Trust a mortgage on the farm.  Aaron transfers ownership of the farm to Bob and gives Bob a general warranty deed.  The general warranty deed to Bob warrants that there are no encumbrances on the farm and does not mention the mortgage to First Bank and Trust.  Bob owns the farm only a short time and sells the farm by limited warranty deed to Carol.  Carol decides that farm life is not for her, and sells the farm by general warranty deed to David.  The general warranty deed does not mention the mortgage to First Bank and Trust.  One week after David purchased the farm, First Bank and Trust notifies David of the mortgage.  

QUESTIONS:
Is there a breach of any deed warranty or covenant, and, if so, which covenant?
If a covenant has been breached, was it a present or a future covenant?
Explain the difference.  At this stage, does David have any basis upon which to sue Carol?
Does David have any basis upon which to sue Bob?  Does David have any basis upon which to sue Aaron?  Explain your answers fully

MY ANSWER:
The transfer of land from Aaron to Bob by general warranty deed has covenant against encumbrances; an absolute warranty against encumbrances. Since there is a mortgage present there is a breach of covenant against encumbrances. This is present covenant allowing Bob to sue Aaron for breach of covenant against encumbrances. The sale of land from Bob to Carol by limited warranty deed then Bob is not responsible for any easements that may arise since he purchased the land from Aaron who is the first grantor. Under the limited warranty deed the grantor is protected against any easements since the easement was created under Aaron's ownership. Later when Carol sold the farm by giving a general warranty deed to David and when the mortgage company contacted David does have a right to sue Carol, since he is protected under the general warranty deed and Carol would be held responsible regardless if she was the cause or creation of the easement was created by a predecessor in title.