What is the binary (base-2) equivalent for the decimal number 9?

Write 9 as a sum of powers of 2: 9 = 8 + 1 = 2³ + 2⁰. The digits for 2³, 2², 2¹, and 2⁰ are therefore 1, 0, 0, and 1, giving 1001 in binary. Choice B, 1001, is correct.

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