\textbf{174-} A diverging lens forms a virtual image 2 cm long from an object 4 cm long. If the object is placed 30 cm from the lens, the actual image distance is 8 cm. What is the focal length of this lens?
\hfill (1) $10$ \hfill (2) $5$ \hfill (3) $4$ \hfill (4) $2$