= Solution
The two historically dominant commercial processes are \b[<gaseous diffusion enrichment> and <gas centrifuge enrichment>]. The first exploits differences in molecular transport through a barrier; the second exploits mass-dependent redistribution in a rotating gas. Both are isotope-separation principles rather than chemical separation of <uranium> from another element.
\b[<gas centrifuge enrichment> had become predominant by the time of this examination.] Its much smaller electricity demand reduces a major operating cost of <diffusion> technology. Modular capacity and the replacement of ageing <diffusion> plants contributed to commercial adoption. This historical conclusion does not require machine dimensions, operating conditions or a cascade design. The United States' final operating <diffusion> plant closed in 2013, consistent with the earlier transition rather than a claim based on current plant counts.<Backgrounder on Uranium Enrichment by the United States Nuclear Regulatory Commission>[Institutional historical sections on gas centrifuges, electricity demand and diffusion-plant closures]{ref}
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