Which observation led Mendel to the idea that traits were controlled by more than a single factor?

Question

Responses

Crossing two purple-flowered pea plants always resulted in a purple-flowered pea plant.
Crossing two purple-flowered pea plants always resulted in a purple-flowered pea plant.

A trait that was not seen in the first generation was completely gone and never reappeared.
A trait that was not seen in the first generation was completely gone and never reappeared.

Crossing two white-flowered pea plants always resulted in a white-flowered pea plant.
Crossing two white-flowered pea plants always resulted in a white-flowered pea plant.

A trait that was not seen in the first generation may later reappear in the second generation.
A trait that was not seen in the first generation may later reappear in the second generation.