Because they occurred one after another, they happened repeatedly.
        
             
        
        
        
Genus
The complete and correct hierarchy of taxonomic groups from largest to smallest are:
1) Domain
     1.1) Archaea  1.2) Bactera  1.3) Eukarya
2) Kingdom
     2.1) Animalia  2.2) Plantae  2.3) Fungi  2.4) Protista  2.5) Archaea  2.6) Bacteria
3) Phylum - has 35 phyla
4) Class 
5) Order
6) Family
7) Genus
8) Species
        
             
        
        
        
Models are simplified representations of of different complex systems. They also help to study very small or large objects. 
Option B, D and E. 
Explanation:
Models are very simplified representations of complex systems and organs or structures which are prepared to study those systems by scientists and normal people easily. Many structures of human body like cells or some tissues are very small in structure or microscopic in nature. So studying them are very hard without specific instruments which are not always available. Thus models are made to easily study those structures by scientists and even normal people and students. 
 
        
             
        
        
        
Answer:
Azidothymidine
Explanation:
Azidothymidine (AZT) is one of the nucleoside analogs that is used in the treatment of AIDS as it inhibits the process of reverse transcription of HIV (Human immunodeficiency virus). 
The HIV replicates by making DNA copies of RNA through the process of reverse transcription. The process is driven by enzyme reverse transcriptase. Azidothymidine (AZT) serves to inhibit the activity of reverse transcriptase enzyme and thereby does not allow HIV to reproduce.
 
        
             
        
        
        
The root tip allows the plant to grow deeper into the soil