Which trait shared by dolphins and bats possibly lead to the evolution of echolocation in these two animal groups?

the need to move quickly through dark environments

Explanation:

Dolphins and Bats are those  creature who uses echolocation in the path which is one property which give them edge in night because they can easily locate things which any other creature face difficult to locate.  

In echolocation what bat and dolphins do is the emit ultra sonic waves which gets deflected from the object and from the deflection they get to know the position of the objects.  

c.the charges if the proton and electron and gravity

small = small current large mag large current