The Biological Classification (Linnean) system includes:
Kingdom
Phylum
Class
Order
Family
Genus
Species 
The best way to remember these is to create your own Mnemonic device. The one I used to memorize these (because you should) was “Kings Play Cards On Fat Green Stools.” This is a very commonly used mnemonic device.
        
                    
             
        
        
        
<span>The enzyme is: ATP synthase.
ATP synthase is an enzymatic protein complex found in the mitochondrial ridges, the thylakoid membrane, and the plasma membrane of bacteria and archaea. The role of this membrane protein is to synthesize adenosine triphosphate (ATP) from the proton electrochemical gradient of protons maintained by the respiratory chain and adenosine diphosphate (ADP), as well as inorganic phosphate, according to the following reaction:ADP + Pi → ATP.
The synthesis of ATP is based on energy conversion, via conformational changes of the subunits.<span>A proton gradient on either side of the membrane into which the ATP synthases are inserted is necessary for their operation, which implies that the synthesis of ATP can not be done independently of a membrane. Within the mitochondrial inner membrane, it is the respiratory chain that provides the pH gradient by injecting protons into the intermembrane space during the transfer of electrons from one complex to another.</span></span>
        
             
        
        
        
Answer: The virus weakens as time passes.
Explanation:  The virus will weaken continuously during transmission, and  will reduce its toxicity after several generations of transmission, without causing much harm to the human body. 
 
        
             
        
        
        
Answer:
a.  Stabilizing selection
Explanation:
Babies lower than 5.5lbs would suffer from various disorder because of low birth weight as their body would not be able to compete as well they would have weak immune system and malnourishment. Whereas high birth weight carry the equally above risk as their body wont be able to cope up and organs become insensitive to various hormones. for the survival the birth weight should be within a median range so that it would be favorable for themselves and surroundings. 
Stabilizing selection occurs when a population stabilizes on a particular trait value and genetic diversity decreases. As in this example, an average baby weight has to favour against extreme variation for it's better survival.
therefore, this is an example of stabilizing selection.