Green pod is the dominant trait, and yellow pod is the recessive trait for pod color in pea plants. What did Mendel discover in his first round of experiments when he crossed true breeding plants with green pods with true breeding plants with yellow pods?

A. All of the offspring had green pods, and the yellow pod trait seemed to disappear.

B. Half of the offspring had green pods, and the other half had yellow pods.

C. All of the offspring had spotted yellow and green pods.

D. One-fourth of the offspring had yellow pods, and three-fourths had green pods.