(a)$(\mathrm{P})=$ purine bases, $(\mathrm{Q})=$ pyrimidine bases, $(\mathrm{R})=$ nucleotides, $(\mathrm{S})=$ nucleosides
(b)$(\mathrm{P})=$ nucleosides, $(\mathrm{Q})=$ nucleotides, $(\mathrm{R})=$ pyrimidine bases, $(\mathrm{S})=$ purine bases
(c)$(\mathrm{P})=$ nucleosides, $(\mathrm{Q})=$ nucleotides, $(\mathrm{R})=(\mathrm{S})=$ purine bases
(d)$(\mathrm{P})=$ nucleotides, $(\mathrm{Q})=$ nucleosides, $(\mathrm{R})=$ pyrimidine base, $(\mathrm{S})=$ purine base