Let $f ( x , y ) = \sqrt { x ^ { 2 } + y ^ { 2 } }$

Multiple Choice

Let $f ( x , y ) = \sqrt { x ^ { 2 } + y ^ { 2 } }$ . Then the maximum value of the directional derivative $D _ { \mathrm { u } } f ( 2,4 )$ is

A) $\sqrt { 3 }$
B) 2
C) 3
D) 1
E) 5