It Was Important That Mendel Examined Not Just the F₁

It was important that Mendel examined not just the F₁ generation in his breeding experiments, but the F₂ generation as well, because

A) he obtained very few F₁ progeny, making statistical analysis difficult.
B) parental traits that were not observed in the F₁ reappeared in the F₂.
C) analysis of the F₁ progeny would have allowed him to discover the law of segregation, but not the law of independent assortment.
D) the dominant phenotypes were visible in the F₂ generation, but not in the F₁.
E) many of the F₁ progeny died.