Let $z = x y + x ^ { 2 } y$

Multiple Choice

Let $z = x y + x ^ { 2 } y$ , and let $x$ and $y$ be functions of $S$ and $t$ with $x ( 0,0 ) = 1 , y ( 0,0 ) = 2$ , $\delta x / \delta s = 3$ , and $\delta y / \delta s = 4$ at $( s , t ) = ( 0,0 )$ . Find $\delta z / \delta s$ when $( s , t ) = ( 0,0 )$ .

A) 21
B) 22
C) 23
D) 24
E) 25
F) 26
G) 27
H) 28