Determine whether the following properties can be satisfied by a function that
is continuous on \((-\infty, \infty) .\) If such a function is possible, provide
an example or a sketch of the function. If such a function is not possible,
explain why.
a. A function \(f\) is concave down and positive everywhere.
b. A function \(f\) is increasing and concave down everywhere.
c. A function \(f\) has exactly two local extrema and three inflection points.
d. A function \(f\) has exactly four zeros and two local extrema.