What distance was used to define the astronomical unit (au)?

This large tectonic plate is bounded on three sides by whats know as the ring of fire. what is the name of this tectonic plate? a) pacific plate b) eurasian plate c) north american plate d) indo- australian plate plz it's science but there's no option for science so i picked chemistry

Why do cells appear different in distilled water than they do in 10% salt water?

What is a possible quantum number set for an electron in the 3s orbital of a magnesium atom