What do the elements mg, ca, and sr have in common

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:

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.