Read the following passage.

A. It sounds like a great idea: experimentally mutate a rare but deadly virus so that scientists can do a better job of recognizing dangerous emerging strains. But it also sounds like a terrible idea — the studies could create a virus that is easier to transmit and produce findings that are useful to bioterrorists. Last year's news that two research teams had done exactly that with the H5N1 bird flu virus was enough to spread fear around the globe and prompt a temporary moratorium on the work. A US biosecurity panel has since lifted its restrictions on publication of the teams' findings in Nature and Science, arguing that the work has clear potential benefits, that the modified virus seems to be less lethal than the original and that the data are already circulating in the community. But the episode has highlighted how thin the line can be between research that's a blessing and research that's a threat.

B. Such fraught lines of enquiry exist in many scientific fields. Some could undermine global security, whereas others could create painful ethical dilemmas for families. The four examples Nature profiles here are hardly a definitive list, but they do give a sense of how frequently such conundrums arise — and show that scientists must constantly ask themselves whether the benefits outweigh the risks. A technology that could quickly and efficiently separate radioisotopes for nuclear power plants and nuclear medicine is one that many physicists might find irresistible. But isotope separation is also key to making nuclear weapons, so such a technology could make it easier both to perform and to conceal illicit work on such weapons.

C. Naturally occurring uranium ore is mostly uranium-238, which cannot sustain the kind of runaway chain reaction required to produce an explosion. Just 0.7% is fissile uranium-235. Enriching that quantity to 3–5% makes fuel for reactors. To make a bomb, it must be enriched to more than 90%. Because the chemistry of the various isotopes is almost identical, sorting one from another has always been one of the major barriers to the proliferation of nuclear weapons. Today's state-of-the-art technology involves cascades of thousands of centrifuges, and so requires space, a massive amount of electricity, precision-machined parts and time.

D. Lasers can be more efficient. Tiny differences in the mass of uranium nuclei alter the energy levels of their electron shells. Finely tuned lasers can excite just the levels associated with the desired isotope and, together with other technology, can sort the uranium-235 from the rest. The work can be done quickly and secretly. In 2004, it emerged that scientists in South Korea had used lasers to enrich small quantities of uranium-235 to near weapons purity in a matter of weeks. The work went undetected for years before it was eventually disclosed to international inspectors.

E. Not given in any of the above paragraphs.