The correct answer is "motor unit - motor neuron".
Explanation:
Motor unit and motor neuron are related terms, however they could not be used interchangeably because a motor unit is not comprised only of a motor neuron. A motor unit is made of a motor neuron, skeletal muscle fibers and neuron's axonal terminals. Therefore motor unit is a more broadly term, and is referred to the units that coordinate the contractions of single muscles.
Isotonic solution This is a solution in which there is no net movement of water into our out of the cell when a cell is placed in this solution. The solution is also dilute.
