Let $L$ be the point $(t, 2)$ and $M$ be a point on the $y$-axis such that $L M$ has slope $-t$. Then the locus of the midpoint of $L M$, as $t$ varies over all real values, is\\
(A) $y = 2 + 2 x ^ { 2 }$\\
(B) $y = 1 + x ^ { 2 }$\\
(C) $y = 2 - 2 x ^ { 2 }$\\
(D) $y = 1 - x ^ { 2 }$