OG10-174解释中的疑问

174.
            A letter by Mark Twain, written in the same year as The Adventures of Huckleberry Finn were published, reveals that Twain provided financial assistance to one of the first Black students at Yale Law School.

(A) A letter by Mark Twain, written in the same year as The Adventures of Huckleberry Finn were published,

(B) A letter by Mark Twain, written in the same year of publication as The Adventures of Huckleberry Finn,

(C) A letter by Mark Twain, written in the same year that The Adventures of Huckleberry Finn was published,

(D) Mark Twain wrote a letter in the same year as he published The Adventures of Huckleberry Finn that

(E) Mark Twain wrote a letter in the same year of publication as The Adventures of Huckleberry Finn that

In this sentence, the relative pronoun that should introduce the clause The Adventures... published to make a relative clause modifying year.
        Also, the singular title of the novel demands a singular verb: for example, one would say, “The Adventures of Huckleberry Finn is (not “are”) a great book.” Only C, the best choice, satisfies both requirements. Choices A and D incorrectly substitute as for that to introduce the relative clause. Choice A also mistakes the novel title for a plural (were published). B confuses meaning (written in the same year of publication as). E creates a similar confusion of meaning, and both D and E are awkward and imprecise because that is too far away from its referent (letter) to be clear.

解释的意思是说，即使A改成，

A letter by Mark Twain, written in the same year as The Adventures of Huckleberry Finn
            was published,也是错的；

我的问题是，为什么一定用that不能用as呢，是因为前面有in么，

如果去掉in the same year中的in，像下面两句话，

That is the same year as
            The Adventures of Huckleberry Finn was published,

That is the same year that
            The Adventures of Huckleberry Finn was published,

也是第二句对，第一句错么？

请同学解答，谢谢；