The immune system, and the excretory system
(Honestly, all of them work to keep us 'healthy', but I believe these are the ones you're looking for)
Answer:
c. environmental factors such as soil pH
Explanation:
Hydrangea plants do exhibit or take on different forms of colors in reaction to environmental factors most especially, the soil pH.
Generally, Hydrangea plants will produce blue flowers if the soil pH level is below 6, while it would produce pink flowers if the pH of the soil is alkaline in nature (pH value is above 7).
The production of Hydrangea plants on the large flower garden, which produces blue flowers and pink flowers, can simply be explained to be as a result of the sensitivity to soil pH. This results in the change of the color of the flower pigment.
It was called the Theory of Inheritance of Acquired Characteristics