In the DNA molecule                 [2008]

(a) The total amount of purine nucleotides and pyrimidine nucleotides is not always equal

(b) There are two strands which run parallel in 5'$\to$3' direction

(c) The proportion of adenine in relation to thymine varies with the organism

(d) There are two strands which run antiparallel one in 5'$\to$3' direction and other in 3'$\to$5' direction