The three major kinds of stimulus that triggers the endocrine glands to synthesize and release hormones are humoral stimuli, hormonal stimuli and neural stimuli. The humoral stimulus regulates the release of hormones with the changes in the concentration of extracellular fluids. For example, the increase of glucose concentration in blood causes pancreas to release hormone insulin.
The hormonal stimuli allows secretion of hormone only with the secretion of any other hormone. The hypothalamus gland secretions allows the anterior pituitary gland to secrete hormones. The release of thyroid hormone is an example of hormonal stimuli.
The neural stimuli allows the production of hormones directly from the glands. The production of epinephrine and norepinephrine hormones are the example of neural stimuli. Thus, option C is correct.
<em>Zebra mussels have been known to have a lifespan of six or seven years in the past but recent discoveries have shown that their average lifespan now ranges between one to two years. Hence, the correct word to fill in the gap would be 'Although'. </em>
