A solution of an optically active compound has specific rotation $+24.0$ (units: degree dm${}^{-1}$ (g/mL)${}^{-1}$). It is placed in a polarimeter tube 2.0 dm long at concentration 0.30 g/mL. Using $\alpha =[\alpha ]lc$, the observed rotation is

1. A$+7.2^{\circ }$
2. B$+48.0^{\circ }$
3. C$+14.4^{\circ }$
4. D$+144^{\circ }$