B. **Confined:** If the study takes a conventional approach and simply explains the reasons behind union growth and decline without delving into broader or more complex aspects, it can be described as confined. The study limits itself to a specific scope without branching into diverse themes or perspectives.

E. **Circumscribed:** Similar to "confined," "circumscribed" suggests a limitation in terms of scope or extent. If the study is primarily focused on explaining the chronological and thematic aspects of union history without exploring a more extensive range of factors, it can be considered circumscribed.

Options A, C, D, and F introduce concepts of complexity, diversity, or difficulty that do not align with the idea presented in the sentence. The sentence emphasizes a straightforward and conventional approach to American union history, making options B and E the most appropriate choices.

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