Answer:
c. mid-adulthood
Explanation:
This stage of life when gray hairs  become overwhelming in number and he has more gray hairs than brunette( brown hair) is called middle adulthood. Middle adulthood spans from 20 to 40 years of life which may be defined differently for different culture and country. 
Robert is in his late mid adulthood somewhere near 40 at this age hair generally grow grey. 
 
        
             
        
        
        
Answer:
The correct answer would be never due to the tilt of the earth.
 
        
             
        
        
        
It is an organic substance.
        
             
        
        
        
- Thermosphere - Fourth layer of Earth's atmosphere.
- Aurora Borealis - Ionosphere layer of thermosphere.
- Large-scale air turbulence and mixing at this layer - Troposphere
- Stratosphere - Second lowest layer of Earth's atmosphere. No Air turbulence so airplanes fly in this layer.
- The coldest temperatures in the atmosphere occur at the junction- of Stratosphere and mesosphere
- Maximum ozone - Ozone layer in Stratosphere
Explanation:
Atmosphere is formed by mixing of gases. The layers of atmosphere is given below.
1. Troposphere - 
- The first layer of the atmosphere.
- Weather occurs in this layer.
2.Stratosphere - 
- The second layer of Earth's atmosphere. 
- Contains Ozone layer.
3. Mesosphere - 
- The third layer above the stratosphere. 
- Coldest layer.
4. Thermosphere - 
- The fourth layer of the atmosphere. 
- Warmest layer.
- Ionosphere - lowest level of thermosphere above mesosphere. 
- Aurora Borealis, Northern and Southern lights are seen in this layer.
- Exosphere - outermost layer of thermosphere.
- Satellites are in this layer.
- Outer space with no gravity.
 
        
             
        
        
        
The gravity is changing in space so everything moves around