The fifth-order partial derivative 5 /x 2 y 3 is zero for each of the following

The fifth-order partial derivative ϑ⁵ƒ/ϑx²ϑy³ is zero for each of the following functions. To show this as quickly as possible, which variable would you differentiate with respect to first: x or y? Try to answer without writing anything down.

a. ƒ(x, y) = y²x⁴e^x + 2

b. ƒ(x, y) = y² + y(sin x - x⁴)

c. ƒ(x, y) = x² + 5xy + sin x + 7e^x

d. ƒ(x, y) = xe^y2/2