Which unstable element is used to determine the age of volcanic rock

Kara has a plant that produces either purple or white flowers. she crosses a plant that has two recessive alleles for white flowers with a plant that has two dominant alleles for purple flowers. which result would be true of all of the offspring? a) all of the offspring would have two recessive alleles and white flowers. b) all of the offspring would have two dominant alleles and purple flowers. c) all of the offspring would have one recessive allele, one dominant allele, and white flowers. d) all of the offspring would have one recessive allele, one dominant allele, and purple flowers