As the fruit develops, why do the petals, stamen, and sepals fall off?

Petals, stamen, and sepals fall off because they are no longer needed by the plant.

Explanation:

Petals, stamen, and sepals are closely related to plant reproduction. Petals protect the reproductory parts of the plant and because the fruit needs to be funded by pollen they no longer need reproductive parts, they have achieved it and now are in the development stage to mature and provide seeds and create more plants. Stamen and sepals are the reproductory parts of the plant and they are no longer needed o achieve reproduction

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms