Take the size of a Rydberg atom to be the diameter of the orbit of the excited electron. If the researchers want to perform this experiment with the rubidium atoms in a gas, with atoms separated by a distance 10 times their size, the density of atoms per cubic centimeter should
be about (a) $10^{5}$ atoms $/ \mathrm{cm}^{3}$; (b) $10^{8}$ atoms $/ \mathrm{cm}^{3}$;
(c) $10^{11}$ atoms $/ \mathrm{cm}^{3}$;
(d) $10^{21}$ atoms/cm $^{3}$.