20. When the hydrogen supply in the core begins to run out, and the star is no longer generating heat by nuclear fusion, the core becomes unstable and contracts. The outer shellof the star, which is still mostly hydrogen, starts to expand. At this phase the star is called a . a. White dwarf
b. Red dwarf
c. Red Giant
d. Black hole

b

Explanation:

i seached it up

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation:

a compound is a molecule made of atoms from different elements.

a. by the combination of an unlimited number of elements