Use the graph of $f$ in the figure to find the following values or state that they do not exist. If a limit does not exist, explain why.
a. $f(1)$
b. $\lim _{x \rightarrow 1} f(x)$
c. $\lim _{t \rightarrow 1^{-}} f(x)$
d. $\lim _{x \rightarrow 1} f(x)$