Approximately 25% of the F2 offspring from Mendel's second set of
experiments expressed a different version of the studied trait than their
parents had. Which of the following best describes this set of experiments?

A. Mendel allowed the offspring from his first set of experiments to
self-pollinate.

B. Mendel observed crosses that took place naturally in his gardens.

C. Mendel crossed plants that all expressed the same version of
each trait.

D. Mendel allowed true-breeding plants to self-pollinate.