Why do all historical accounts, even history books, include some form of subjectivity when they are supposed to be objective?'

because they had to be written by people who were there, and those people almost always will be biased in some form or another.

when making an inference, the reader should be sure to support the inference with

evidence

a is an educated guess about what will happen.

c. prediction

when new information proves that a reader's prediction is wrong, the reader should

make a new prediction based on the new information.

what excerpt? i would generally think a.