astrophysicists study the stars and other celestial bodies. during their observations, they can only see the stars exploding, but they cannot hear it, even with powerful microphones. what could be the best explanation for this?

According to the article, what valid claim came from the national research council? there is no association between fluoride exposure and increased cancer risk in humans. there is a possible link between fluoride exposure and increased cancer risk in humans. there is a definite link between fluoride exposure and increased cancer risk in humans.