Which expression gives the correct volume of the steps?

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4×4×1 cubic feet



(4×4×1)+(3×3×3) cubic feet



(4×4×1)+(3×3×1)+(2×2×1)+(1×1×1) cubic feet



(4×4×4)+(3×3×3)+(2×2×2)+(1×1×1) cubic feet