Gregor Mendel was the father of genetics. When experimenting with tall and short pea plants, he discovered that tall pea plants were dominant over short pea plants. What was the evidence that helped him determine this discovery?

a) The first generation of pea plants were medium sized plants
b) The first generation of pea plants were all tall.
c) The second generation had all short plants.
d) All the first generation of pea plants were a mix with short and tall pea plants.