Correct: (B.) she supposed was
The salesgirl showed her several bags and she took the one she supposed was the most smart.
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Yes, there is a difference between A and B.
Two persons on this sentence: a salesgirl, and the other girl. The subject was the salesgirl. If using "that," the "that" is referring to the subject - salesgirl.
If choosing " (A.) that she supposed was," the "that she" would be talking about the salesgirl, instead of the other girl. The sentence would become very confusing.
Therefore, the correct answer is (B.)
Alex :~)
参考资料：||
按严格的语法来讲 B答案是正确的。当先行词为特指时，如the one, the girl, 这时的that 是不能省略的。另外当先行词有most, only等词修饰时，也不能省略that.
这两个选项本质上没有区别，都可以。但考是一般考比较正式的用法。A项that是定语从句的象征，传统语法中一般都不能省略，考试时出题人员主要考察学生对定语从句引导词和从句中句序的用法，所以通常情况下必须选A.B项是口语用法。如果不是从考试的角度出发，两个都是正确的。
