I thought it was obvious from the text but...
a) The logic is that if animal sacrifices could remove sins then why did it need to be done every year? Surely the fact that it was done every year disproves that theory. Now, one can drive the proverbial coach and horses through that, as I am sure you will see, if you wish to, but it must be balanced with that which was in the mind of the writer at the time...
b) Yeshua's once only sacrifice serves to cover all sin for each individual on an 'as needed' basis, for want of a better term. Therefore, for a Christian, provided that you have truly repented of your individual sin (which is a violation of G_d's law ), then the price for that sin has already been paid by Yeshua giving his body on the cross. G_d has provided his son to be a once only, final blood sacrifice. Grace is the name of the forgiveness that G_d gives because of Yeshua, as he wipes the sin from the book of your life at judgement day (Revelation). However, very few Christians see the fact that grace only works in relation to unintentional violations the Law. Grace with out the Law (prevenient grace) is a nonsense.
I'll leave it there at the moment.