When two bulbs of power $60 \mathrm{~W}$ and $40 \mathrm{~W}$ are connected in series, then the power of their combination will be (a) $100 \mathrm{~W}$ (b) $2400 \mathrm{~W}$ (c) $30 \mathbf{W}$ (d) $24 \mathrm{~W}$