Biochemistry 6th Edition by Reginald Garrett,Charles Grisham

Exercise 6

Solving the sequence of an oligopeptide from sequence analysis data
Amino acid analysis of an octapeptide gave the following results:
The following facts were observed:
a. Trypsin treatment yielded a pentapeptide and a tripeptide.
b. Chemical reduction of the free ? -COOH and subsequent acid hydrolysis yielded 2- aminopropanol.
c. Partial acid hydrolysis of the tryptic pentapeptide yielded, among other products, two dipeptides, each of which contained C-terminal isoleucine. One of these dipeptides migrated as an anionic species upon electrophoresis at neutral pH.
d. The tryptic tripeptide was degraded in an Edman sequenator, yielding first A, then B:

What is an amino acid sequence of the octapeptide? Four sequences are possible, but only one suits the authors. Why?