Which of the following is not a characteristic of trap$-$door one$-$way functions?

Select one:

a$.$ Easy to compute $Y=f_k(x)$ when $k$ and $x$ are known

b$.$ Infeasible to compute $x=f_k^{-1}(Y)$ when $Y$ is known but $k$ is not known

c$.$ Easy to compute $x=f_k^{-1}(Y)$ when $k$ and $Y$ are known

d$.$ Easy to compute $Y=f_k(x)$ when $k$ is not known