Answer: The "Forchhammer" principle
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BF3 acts as a Lewis acid when it accepts the lone pair of electrons that NH3 donates.
(2) U-238 is the radioisotope used in dating geological formations. Uranium-238 has a half-life of 4.46 billion years, which makes it suitable to date such old formations.
The other options do not have such long durabilities and half-lives.