Find a Mathematical Model Representing the Statement $( z = 128 \text { when } x = 4 \text { and } y = 8 . )$

Multiple Choice

Find a mathematical model representing the statement.(Determine the constant of proportionality.)
Z varies jointly as x and y. $( z = 128 \text { when } x = 4 \text { and } y = 8 . )$

A) $z = \frac { 4 y } { x }$
B) $z = \frac { 4 } { x y }$
C) $z = \frac { x y } { 4 }$
D) $z = \frac { 4 x } { y }$
E) $z = 4 x y$