62. The fifth-order partial derivative a³f/ðx²ay³ 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. f(x, y) = y²x'e* + 2 b. f(x, y) = y² + y(sin x – x*) c. f(x, y) = x² + 5xy + sin r + 7e* d. f(x, y) = xe³/2 %3D