함수 $w=x ^ {2021} +xy+yz, x= \sin s+ \cos t, y=e ^ {s-t}, z=s+t$ 일때, $\displaystyle \left. \frac{\partial w} {\partial s} \right | _ {s=0, t=0}$ 은?

• ①$2021$
• ②$2022$
• ③$2023$
• ④$2024$