A. Nile river <em>is the answer</em>
Answer: Muscle tone
Explanation: Muscle tone is actually a muscle tension. This tension (muscle tone) maintains the body in its position as it is, or in a particular desired position. Or, for example, during movement, when the body changes its position, these changes of position are regulated by muscle tone. When the body is at rest, some muscle tension exists, and depending on the general fitness of the body, that tension under the skin is more visible or less visible, that is, my muscles will be tighter or floppy.
