In your own words, what is...
a) Beat
b) Rhythm

in my opinion

Explanation:

Beat is  tempo

Rhythm is a repeated pattern of movement or sound

in this piece, the rectangles, the vertical and horizontal lines, and lack of hue create a sense of harmony. there is balance and proportion because although the rectangles are different, no one rectangle overpowers the others, and the size relationship among the rectangles is agreeable to the eye. there is a sense of movement and depth, through the use of shading and shadowing. it almost appears like some of squares are popping out. there is variety created by the different rectangle sizes, the shading, and the shadowing.

aka im the smartest of them all