Answer:
D) hot stars and interstellar gas, particularly hydrogen
Explanation:
Emission nebula is the nebula which is formed of the ionized gases which emit light of different wavelengths. The common source of ionization of the emission nebula is high-energy photons mostly which are emitted from hot star present nearby to it. 
<u>The emission nebulae include H II regions. An H II region is the region of the interstellar atomic hydrogen which is ionized. It is the cloud of the partially ionized gas in which star formation takes place.</u>
 
        
             
        
        
        
<h3>answer</h3>
The minimum velocity required for a object to rotate in a verticle plane is v 
1
 
 = 
5
 
 gr.
So, at bottom point, T 
max
 
 =mv 
1
2
 
 /r+mg=6mg.
At top-most point velocity is v 
2
 
 =gr.
So, T 
min
 
 =mv 
2
2
 
 /r−mg=0
So T 
max
 
 −T 
min
 
 =6mg=2×g⇒m=1/3 kg
 
        
             
        
        
        
Answer:
B. When the racket hits the tennis ball with a force, the tennis ball applies an equal but opposite force to the racket.
Explanation:
According to the Newton's third law of motion every action has equal and opposite reaction. So, when the force is applied by the racket on the ball then the ball also applies an equal intensity of force in the opposite direction on the racket. It is just that the the force on the racket is absorbed by the player holding it.
 
        
             
        
        
        
Answer:
5 seconds
Explanation:
<em>Acceleration = (final velocity - initial velocity) ÷ time</em>
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