The main difference between stream pools and ponds is that stream pools

a. have an observable current
b. are located inland
c. have no access to rivers or streams
d. are usually larger than ponds

The correct answer is option A, have an observable current

Explanation:

It is the stream pools that have an observable current

Stream pools are basically the water segments found below a water fall, spring or any flowing water form. The pool possess an average speed due to the water entering and leaving with certain speed. Due to the speed of water,  water currents are  induced in the stream pool. It is usually found at the downstream of any water body where the water speed reduces than the actual while in a pond the water currents remain stagnant and thus there is no observable water current in it.  

do you have a picture?

littering streams           is the answer