About 300 b. c., aristarchus suggested that the sun was at the center of the universe. why?

the brightness of the planets would change.
the planets appeared to move among the constellations.
the planets always moved opposite to the stars.
the planets disappeared periodically.

The correct answer is option A and B

About 300 B.C., Aristarchus suggested that the sun was at the center of the universe because a) the brightness of the planets would change and b) the planets appeared to move among the constellations.

18 centuries before  Copernicus, Aristarchus predicted that  all planets orbit the sun. In other words, he suggested the Solar System is heliocentric.

the grams   of   barium that   are in   2.3 x 10^23   atoms of barium   is   52.4 g (answer c)

    calculation

mass = moles   of ba   x molar   mass of ba

  molar mass of ba from periodic table=   137.3 g/mol

  use the   avogadro's law   constant   to   calculate the moles of barium

  that   is     1 mole =   6.02   x 10^23 atoms

                    ? moles = 2.3 x 10^23   atoms

by cross   multiplication

=(1 mole   x 2.3x 10^23 atoms)   / 6.02   x10^23 atoms)=   0.382   moles

mass   is therefore = 137.3   g/mol x 0.382 mol = 52.4 ( answer c)

explanation: