What is the uses of echoes?

What is a neurotransmitter involved in mood reward, addiction, and motor behaviors?

Individuals who live below the poverty line get seriously ill more often than those who do not what could be the hidden variable in this situation?

Students design a model roller-coaster track. they place a rubber ball at the highest point on the track and let it go. the ball rolls along the track pulled only by the force of gravity. eventually, it comes to a stop. which change to the design will result in the ball moving the greatest distance?