On which line is the first error in the following argument?
A $\sin ^ { 2 } x + \cos ^ { 2 } x = 1$ for all values of $x$.\\
B Therefore $\cos x = \sqrt { 1 - \sin ^ { 2 } x }$ for all values of $x$.\\
C Hence $1 + \cos x = 1 + \sqrt { 1 - \sin ^ { 2 } x }$ for all values of $x$.\\
D Thus $( 1 + \cos x ) ^ { 2 } = \left( 1 + \sqrt { 1 - \sin ^ { 2 } x } \right) ^ { 2 }$ for all values of $x$.\\
E Substituting $x = \pi$ gives $0 = 4$.