Answer: 4.
This is a free search engine maintained by the United States National Library of Medicine.
It is used for accessimg and searching of MEDILINE database.
Medline is an acronym for (Medical Literature Analysis and Retrieval System Online) it is conprehensive bibilography of research materials in fields of biomedicine and life sciences.It contains abstracts and references of related journals.
It is an online repository for biomedical research materials 
It also helps to promotes the concept of evidence based medicine.(The concept of clincal medicine that depends on the combination of available research evidences with patients values and clincal research.)
Therefore the use of Pubmed is to gain access into all the information above in the MEDLINE.
 
        
             
        
        
        
Homologous stuctures because, homo means same implying that the cell/animal is not change as it continues to reproduce
        
                    
             
        
        
        
Skeletal tissue which makes up the skeletal system.
        
             
        
        
        
Intercalated disks support the synchronized contraction of cardiac muscles
        
             
        
        
        
The three scientific principles of sustainability are: (1) dependence on solar energy, (2) Biodiversity, and (3) chemical cycling.
dependence on solar energy - the energy imparted to the earth system by the sun. The sun warms the planet and provides energy that plants use to produce nutrients. The sun also powers indirect forms of solar energy such as wind and flowing water.
biodiversity -