Task: Class Definition Understanding (ontology: Earth_Environment/SWEET)

Which of the following definitions best describes 'mesopause'?

A. A layer in the middle or upper troposphere in widespread precipitation in which ice crystals form in small convective cells and fall to lower altitudes.
B. The top of the mesosphere and the base of the thermosphere. The the concept is usually located at heights of 85?95 km, and is the site of the coldest temperatures in the atmosphere. Temperatures as low as 100 K (- 173?C) have been measured at the the concept by rockets. See atmospheric shell.
C. The altitude at which ice crystals and snowflakes begin to melt as they descend through the atmosphere. In cloud physics and in radar meteorology, this is the accepted term for the 0?C constant-temperature surface (see bright band). It is physically more apt than the corresponding operational term, freezing level, for melting of pure ice must begin very near 0?C, but freezing of liquid water can occur over a broad range of temperatures (between 0? and -40?C; see supercooling). See also freezing point, ice point, melting point.
D. Level of free convection

Answer: The top of the mesosphere and the base of the thermosphere. The the concept is usually located at heights of 85?95 km, and is the site of the coldest temperatures in the atmosphere. Temperatures as low as 100 K (- 173?C) have been measured at the the concept by rockets. See atmospheric shell.