Why Is It Usually Advantageous to Not Place Key Issues

Why is it usually advantageous to not place key issues of a negotiation at the very beginning of the agenda?

A) It provides an opportunity to learn the other side's bargaining style and concession routines.
B) It saves time as it becomes obvious that if an agreement can't be reached on the smaller issues then there is no point in discussing the key issues.
C) By conceding most of the smaller issues in the beginning,the selling team creates a moral "debt," which the buying team is likely to have to repay by conceding on the key issues.
D) It allows the team setting the agenda to wear down the other side,which will likely result in them agreeing to all the key issues proposed.
E) It provides the sellers an advantage as the buyers lose interest toward the end of the session when the key issues are discussed.