A natural rock formation. Hope this helped.
        
                    
             
        
        
        
the answer your looking for is Optical instrument.
 
        
                    
             
        
        
        
B. the distance the star is from Earth
Explanation:
The apparent magnitude of star is a function of its distance from the earth. It is one of the physical properties that is used to study a star. 
The apparent magnitude of a star or other astronomical bodies is a measure of their brightness as seen from a location on the earth. 
The apparent magnitude of a star depends on: 
- Distance of the star from the location on earth. 
- luminosity of the star
- the particles along the part of the star and earth that cuts off the light the earth receives. 
learn more: 
Star luminosity brainly.com/question/9084808
#learnwithBrainly
 
        
             
        
        
        
German physicist  Albert Betz  (in 1919) demonstrated  that the highest efficiency you can achieve with a wind turbine is around 59%
We would have to analyze the design of an specific turbine to determine its efficiency, however it is unlikely to achieve 50% , as todays turbines have an average efficiency in the 20-35%
The answer would be around 25%