Why is unity sometimes regarded as the most important of all design principles?

Order the events as they occurred in history. thomas edison and william dickson invent the kinetoscope. the view from the window at le gras is created. disney produces flowers and trees, the first color film. stanley kubrick revolutionizes special effects with 2001: a space odyssey. muybridge creates the horse in motion. francis ford coppola’s apocalypse now is selected for preservation by the library of congress. d. w. griffith directs the birth of a nation. marilyn monroe makes her film debut.

Ineed someone to describe what they mean in the instructions !

Artists best achieve contrast in a composition by: a. drawing with realism b. shading a drawing with dark and light values c. using drawing techniques like stippling and hatching d. focusing on line and shape in a drawing