Use the binomial theorem to expand (a + b)6.

By the binomial theorem, (a + b)^6 = Σₖ₌₀⁶ ⁶Cₖ a⁶⁻ᵏbᵏ. The terms run from ⁶C₀a⁶ through ⁶C₆b⁶, so the expansion includes seven terms, with the final term b⁶. Choice C gives all of these terms; choice A leaves off the final term.

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