Answer:
b. 3.34 x 10^-3
Explanation:
According to Avogadro's constant, in one mole, there are 6.02 x 10^23 atoms. To see how many moles are in 2.01 x 10^21 atoms, you would divide it by the amount of atoms that fit in a mole.
2.01 x 10^21 atoms / 6.02 x 10^23 = 0.00334 moles, which can be
converted to 3.34 x 10^-3 moles