设 $z=y^x$，则 $\left(\dfrac{\partial z}{\partial x}+\dfrac{\partial z}{\partial y}\right)\Big|_{(2,1)}=$（ ）

1. A 选项 A 2
2. B 选项 B $1+\ln2$
3. C 选项 C 0
4. D 选项 D 1