If the echo (the reflected sound) reaches your ear less than about 
0.1 second after the original sound, your brain doesn't separate them, 
and you're not aware of the echo even though it's there.
If the echo comes from, say, a wall, 0.1 second means you'd have to be
about  17 meters away from the wall.  If you're closer than that, then the
echo reaches you in less than 0.1 second and you're not aware of it.
A. 30 meters . . . 
     No.  You hear that echo easily
B.  you're standing within range of both sounds . . . 
     No. You hear that echo easily, if you're at least 17 meters from the wall. 
C.  less than 0.1 second later . . .
     That's it.  The echo is there but your brain doesn't know it.
D.  21.5 meters
     No.  You hear that echo easily.
        
                    
             
        
        
        
Answer:

Explanation:
Formula for AVERAGE acceleration

 is Final Velocity
 is Final Velocity
 is Initial Velocity
 is Initial Velocity
 is Final Time
 is Final Time
 is Initial Time
 is Initial Time
Most of the time, the initial time will be 0, so you won't have to subtract.
 
        
             
        
        
        
Explanation:
Let us calculate the work done in lifting an object of mass m through a height h, such as in Figure 1. If the object is lifted straight up at constant speed, then the force needed to lift it is equal to its weight mg. The work done on the mass is then W = Fd = mgh. We define this to be the gravitational potential energy (PEg) put into (or gained by) the object-Earth system. This energy is associated with the state of separation between two objects that attract each other by the gravitational force
Potential energy is a property of a system rather than of a single object—due to its physical position. An object’s gravitational potential is due to its position relative to the surroundings within the Earth-object system. The force applied to the object is an external force, from outside the system. When it does positive work it increases the gravitational potential energy of the system. Because gravitational potential energy depends on relative position, we need a reference level at which to set the potential energy equal to 0. We usually choose this point to be Earth’s surface, but this point is arbitrary; what is important is the difference in gravitational potential energy, because this difference is what relates to the work done. The difference in gravitational potential energy of an object (in the Earth-object system) between two rungs of a ladder will be the same for the first two rungs as for the last two rungs.
 
        
             
        
        
        
Answer:
Current, I = 0.000109 Amps
Explanation:
Given the following data;
Voltage = 6V
Resistance = 55,000 Ohms
To find the current flowing through the circuit;
Ohm's law states that at constant temperature, the current flowing in an electrical circuit is directly proportional to the voltage applied across the two points and inversely proportional to the resistance in the electrical circuit.
Mathematically, Ohm's law is given by the formula;
 
 
Where;
V represents voltage measured in voltage. 
I represents current measured in amperes. 
R represents resistance measured in ohms. 
Making current the subject of formula, we have;
 
 
Substituting into the formula, we have;
 
 
Current, I = 0.000109 Amps