Mendel Crossed Two Pea Plants with Round Seeds

Mendel crossed two pea plants with round seeds.All seeds of the offspring were round.He then crossed a plant with round seeds to a plant with wrinkled seeds and all offspring had wrinkled seeds.Which of the following is true?

A) wrinkled is dominant
B) the plants he used were not true breeding
C) a mutation occurred
D) the trait does not breed true
E) round is dominant