Which of the following best describes rem sleep?

Sleep in which the brain becomes more active but muscle activity is blocked.

The brain cycles through five distinct phases during sleep: stages 1, 2, 3, 4, and rapid eye movement (REM) sleep. REM sleep makes up about 25% of your sleep cycle and first occurs about 70 to 90 minutes after you fall asleep. Because your sleep cycle repeats, you enter REM sleep several times during the night.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.