I believe it's the ovule.
        
             
        
        
        
The correct answer is HPA axis.
The Hypothalamic-Pituitary-Adrenal (HPA) axis is one of the most important neuroendocrine systems, which regulates the stress response and other functions such as the digestion, mood, emotions and the immune system. 
The hypothalamus, when triggered by a possible stressor, releases two hormones; the vasopressin and the corticotropin-releasing hormone (CRH). CRH, in turn, triggers the release of the adrenocorticotropin hormone (ACTH) from the pituitary gland. As a result of the secretion of ACTH, cortisol is secreted by the adrenal cortex. 
Cortisol is a steroid hormone, considered to be our body's stress hormone. 
        
             
        
        
        
My Answer: <span> lower than terrestrial planet densities, lower than the density of rock.
Hope I helped!  :D
</span>
        
                    
             
        
        
        
The sodium hydroxide solution and vegetable oil underwent a chemical change. 
<h3><u>Explanation:</u></h3>
The reaction between a base and a fat or fatty acid is called as saponification reaction. Here the base dissociates into the cation and hydroxyl ion. The hydrogen from the fatty acid of the oil reacts with the hydroxyl ion to form water which is basically the neutralization reaction. This cation now reacts with the rest of the fatty acid to form a bipolar molecule which is the soap. During the reaction, the solution becomes cloudy but with time, it again goes back to solution. The cloudyness appears because oil isn't soluble in water based solutions and the base is actually made solution in water. 
Here the sodium hydroxide reacts with oil to form water and the soap molecule which is a chemical change.