Which of the following did Gregor Mendel notice in garden peas?

A) Some plants always produced offspring with flowers that were the same color as the parent plants' flowers.
B) The first generation from a genetic cross of true-breeding plants always produced offspring that were intermediate in their phenotype and did not look like either parent.
C) Garden peas always produce a variety of phenotypes in their offspring, which result from frequent mutations.
D) Any individual pea plant carries at least three different alleles for flower color.