Evaluate $\iint _ { S } f ( x , y , z ) d S$

Multiple Choice

Evaluate $\iint _ { S } f ( x , y , z ) d S$ . $f ( x , y , z ) = x + y$ ; S is the part of the plane $7 x + 3 y + z = 21$ in the first octant.

A) 0
B) $\sqrt { 327 }$
C) $35$ $\sqrt { 59 }$
D) $2 \sqrt { 139 }$