Sodium (Na) is very reactive because it does not have a full valence shell.
        
             
        
        
        
Answer : The correct option is, (D) A machine does 400 joules of work in 5 seconds.
Explanation :
Power : It is defined a the rate of doing work per unit time.
Formula used :

where,
P = power
w = work done
t = time
Now we have to determine the rate of power for the following options.
(A) A machine does 200 joules of work in 10 seconds.

(B) A machine does 400 joules of work in 10 seconds.
  
(C) A machine does 200 joules of work in 5 seconds.
  
(D) A machine does 400 joules of work in 5 seconds.

From this we conclude that, a machine does 400 joules of work in 5 seconds has the highest rate of power.
Hence, the correct option is, (D)
 
        
             
        
        
        
Answer:he conducts the investigation to see the effect acidic water has on limestone
Explanation:
 
        
             
        
        
        
At -40. 
-40 gives the same reading for Fahrenheit and Celsius scale.
        
             
        
        
        
Answer:

Explanation:
The intensity is related to the power and surface area by  . We need to calculate the surface area of a sphere of radius r=4.3ly.
. We need to calculate the surface area of a sphere of radius r=4.3ly.
Since 4.3ly is the distance light travels in 4.3 years at 299792458m/s, we can obtain it in meters by doing:

So we have:
