Why Do Plants Growing Outside Often Have Shorter and Thicker

Why do plants growing outside often have shorter and thicker stems than plants of the same species grown indoors?

A) The plants inside have a more constant source of light and therefore a higher level of phytochrome.
B) The outdoor plants are subject to mechanical stresses and they are exhibiting a thigmomorphogenic response.
C) The outdoor plants are subject to touch by animals and other plants and are exhibiting a thigmotropic response.
D) The indoor plants have elevated levels of auxin and are therefore taller than the outside plants.