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/cm$^3$; (b) 10$^8$ atoms/cm$^3$; (c) 10$^{11}$ atoms/cm$^3$; (d) 10$^{21}$ atoms/cm$^3$.