Two AM's $\mathrm{A}_{1}$ and $\mathrm{A}_{2}$, two GM's $\mathrm{G}_{1}$ and $\mathrm{G}_{2}$ and two $\mathrm{HM}$ 's $\mathrm{H}_{1}$ and $\mathrm{H}_{2}$ are inserted between two positive numbers. Then, $\mathrm{H}_{1}^{-1}+\mathrm{H}_{2}^{-1}=$
(a) $\mathrm{A}_{1}^{-1}+\mathrm{A}_{2}^{-1}$
(b) $\mathrm{G}_{1}^{-1}+\mathrm{G}_{2}^{-1}$
(c) $\mathrm{A}_{1} \mathrm{H}_{1}+\mathrm{A}_{2} \mathrm{H}_{2}$
(d) $\frac{A_{1}+A_{2}}{G_{2} G_{2}}$