Gene S controls the sharpness of spines in a type of cactus. Cacti with the dominant allele, S, have sharp spines, whereas homozygous recessive ss cacti have dull spines. At the same time, a second gene, N, determines whether or not cacti have spines. Homozygous recessive nn cacti have no spines at all. If cacti heterozygous for both traits, SsNn, were allowed to self-pollinate, the offspring would segregate into which of the following phenotype ratios?
a) 3 sharp-spined: 1 spineless.
b) 1 sharp-spined: 2 dull-spined: 1 spineless.
c) 1 sharp spined: 1 dull-spined: 1 spineless.
d) 1 sharp-spined: 1 dull-spined.
e) 9 sharp-spined: 3 dull-spined: 4 spineless.