If $k<0$ , What Is the Solution Set Of $|3 f+1|< k$

Multiple Choice

If $k<0$ , what is the solution set of $|3 f+1|< k$

A) $\varnothing$

B) $\left(-\frac{\mathrm{k}+1}{3}, \frac{\mathrm{k}+1}{3}\right)$

C) $(-\infty, \infty)$

D) $\{0\}$