The group in which a star is classified according to its spectrum, especially using the Harvard classification.
        
             
        
        
        
Mitochondria convert the energy in the bonds of glucose and oxygen to readily available energy in the form of ATP (Adinoshin Tri Phosphate) with the help of oxygen. Mitochondria are often referred to as the powerhouse of cell.
        
             
        
        
        
Yes there is, there is the cycle which trees and plant organisms produce oxygen, and homes animal organisms which can be beneficial to decomposition and other things.
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The appropriate answer is c. a scientific journal. A scientific journal is a collection of papers based on topics in science that have been written by experts in their fields. Individuals with a masters or doctorate in various subject areas are published here. 
From the list given this is the best second source for the research. Science fiction novels and not real and so would be unacceptable. Magazines and news papers reporting on scientific research most likely get information from scientific journals.