Let $z = \ln \left( x ^ { 2 } + y ^ { 2 } \right)$

Multiple Choice

Let $z = \ln \left( x ^ { 2 } + y ^ { 2 } \right)$ , where $x = e ^ { t }$ and $y = e ^ { - t }$ .When is

A) $- 2$
B) 0
C) 1
D) 2
E) $- 1$