The "its" without the apostrophe is the possessive form of the pronoun "it" (as in: "The dog slept in its bed." that is, the dog "possesses" the bed).
The "it's" with the apostrophe is a contraction meaning "it is" (as in: "The professor has said that it's a good idea to keep all your returned papers until after the semester's end.").
The confusion of "its" and "it's" is called a "homophonic error" and is considered a major writing error (because it's so simple).
You must correct this error now! In future, this particular homophonic error will earn a "d" (or lower) for the paper, no matter what other good qualities the paper may have.
Yikes!
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