Let $f ( x ) = \left\{ \begin{array} { l l } 2 x + 1 & x < 0 \\- x + 1 & x > 0\end{array} \right.$

Multiple Choice

Let $f ( x ) = \left\{ \begin{array} { l l } 2 x + 1 & x < 0 \\- x + 1 & x > 0\end{array} \right.$ Which of the following is true?

A) $f$ is discontinuous at 0.
B) $f$ has a derivative at 0.
C) $f$ has a corner at (0,0) .
D) $f$ has a has a horizontal tangent line at (0,0) .
E) $f$ has a vertical tangent line at (0,0) .