If a Rectangle Is X Feet Long and Y Feet $x + y$

Multiple Choice

If a rectangle is x feet long and y feet wide, which one of the following expressions is the length of its diagonal in terms of x and y?

A) $x + y$
B) $\sqrt { x + y }$
C) $x ^ { 2 } + y ^ { 2 }$
D) $\sqrt { x ^ { 2 } + y ^ { 2 } }$