Write the names for the elements in each of the following formulas of compounds used in medicine:
a. table salt, $\mathrm{NaCl}$
b. plaster casts, $\mathrm{CaSO}_{4}$
c. Demerol, $\mathrm{C}_{15} \mathrm{H}_{22} \mathrm{ClNO}_{2}$
d. treatment of bipolar disorder, $\mathrm{Li}_{2} \mathrm{CO}_{3}$