In Mendel's Pea Plants,how Did He Know That Each F1

In Mendel's pea plants,how did he know that each F1 generation contained two alleles encoding different characteristics?

A) The F1 generation had a blended phenotype of the two parental phenotypes.
B) Both parental phenotypes reappeared in the F2 generation.
C) Each F1 plant had a different phenotype.
D) The F1 plants did not have two alleles.
E) none of the above