Let $S = \left\{ x - y \mid x , y \text{ are real numbers with } x ^ { 2 } + y ^ { 2 } = 1 \right\}$. Then the maximum number in the set $S$ is (A) 1 (B) $\sqrt { 2 }$ (C) $2 \sqrt { 2 }$ (D) $1 + \sqrt { 2 }$.
Let $S = \left\{ x - y \mid x , y \text{ are real numbers with } x ^ { 2 } + y ^ { 2 } = 1 \right\}$. Then the maximum number in the set $S$ is\\
(A) 1\\
(B) $\sqrt { 2 }$\\
(C) $2 \sqrt { 2 }$\\
(D) $1 + \sqrt { 2 }$.