Which of the following is a true statement? А
Periods form horizontal rows; groups form vertical columns
B Groups form horizontal rows; periods form vertical
C Categories form horizontal rows; gases form vertical
D
Gases form horizontal rows; categories form vertical
columns

not clear

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: