Using a Zippe-type centrifuge, natural uranium can be enriched by the third hour of rotation.


(A) Using a Zippe-type centrifuge, natural uranium can be enriched by the third hour of rotation.
(misplaced; it's not the uranium that is using the centrifuge)

(B) Natural uranium can be enriched by the third hour of rotation, using a Zippe-type centrifuge.
(still misplaced; it's not the third hour of rotation that's using the centrifuge)


(C) A scientist, enriching natural uranium by the third hour of rotation, can use a Zippe-type centrifuge.
(grammatically it might be okay, but it changes the meaning; it now sounds like the scientist is enriching uranium to use the centrifuge rather than using the centrifuge to enrich uranium)

(D) By rotating for three hours and using a Zippe-type centrifuge, natural uranium can be enriched by a scientist.
(misplaced; it's not the uranium rotating itself and using the centrifuge)


(E) Using a Zippe-type centrifuge, a scientist can enrich natural uranium by the third hour of rotation.
(correct; the scientist is using the centrifuge to enrich uranium)




This is a great example of a sentence where we have to change the meaning by adding something (the scientist). From reading the original, we should notice that we do not have a noun that can be modified by the opening phrase ("using a Zippe-type centrifuge"), which means that we have to add it ourselves. As a general rule, preserve the original meaning as much as possible; if it's illogical or impossible, look for a way to fix it.

OA is E