Which choice completes the text with the most logical transition? The Sun and other stars are powered by nuclear fusion reactions, in which two atoms collide to form a single heavier atom, releasing energy. Scientists have long believed that fusion has the potential to meet humanity's clean energy needs. ______blank prior to December 2022, no fusion reaction in a laboratory setting had ever generated a net energy gain.

Choice D is the best answer. Scientists believe in fusion's potential as an energy source, but have struggled to actually make it work-in other words, there is a contradiction between scientists' beliefs and their reality. 'That said' is a disagreement transition that works perfectly in this context. Choice A is incorrect. This choice uses a cause-and-effect transition, which doesn't make sense here. Scientists not being able to generate extra energy from lab fusion reactions isn't an effect of them believing in fusion's potential. Choice B is incorrect. This transition indicates the addition of another supporting point. But this sentence is not adding a supporting point to the previous sentence-scientists not being able to successfully generate energy from fusion isn't another point in favor of fusion meeting humanity's clean energy needs. Choice C is incorrect. This choice uses a transition that introduces or elaborates on a particular example. But this sentence doesn't give an example of scientists' belief in fusion's potential to meet humanity's clean energy needs-in fact, it contrasts that optimistic belief with the reality of past failures to successfully employ fusion for energy production.