How can 2 plants that look the same on the outside produce offspring that look different from them?

They are both heterozygous therefore have a dominant gene (B) showing but have a recessive allele (b).

If Bb and Bb have offspring, they can have outcomes: BB, Bb or bb.
The offspring bb will have a different looking offspring.
Eg.
B: Brown eyes
b: Blue eyes

BB: Brown eyes (homozygous dominant)
Bb: Brown eyes (heterozygous)
bb: Blue eyes (homozygous recessive)

Blue eyes are the recessive gene and will not show unless both alleles are recessive.

And a) oxygen i believe