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I need help with the following sentences. The first and fifth sentences are from The New York Times. Others are slight modifications of the original sentences. I would like to know which of the other forms are acceptable. Please help. Thanks.

1. A nuclear fusion power station is the 'Holy Grail' for scientists trying to find a viable alternative to the world's depleting stocks of oil and gas. (NYT)
2. A nuclear fusion power station is the "Holy Grail" for scientists trying to find a viable alternative to the world's depleting stocks of oil and gas. (Note double quotes instead of single)
3. A nuclear fusion power station is the 'Holy Grail' for scientists WHO ARE trying to find a viable alternative to the world's depleting stocks of oil and gas.
4. A nuclear fusion power station is the 'Holy Grail' for scientists THAT ARE trying to find a viable alternative to the world's depleting stocks of oil and gas.

5. But critics argue it could be at least 50 years before a commercially viable reactor is built, if at all. (NYT)
6. But critics argue THAT it could be at least 50 years before a commercially viable reactor is built, if at all.
Comments  
Hi elviajero

when using a verb form with 'ing'(as in the first sentence above) you can omit 'who+be' and 'that+be' after scientists
Consequently, #3 and 4 are correct and completely acceptable: 'who' refer to persons, such as scientists, and 'that' can also refer to persons though it is more acceptable to use 'who' in this case.
about the double and single quotes, I don't know the differences...
Thank you, Amandine!