The number of distinct fundamental absorption bands observed in the infrared spectrum of a carbon dioxide molecule is

$\mathrm{CO_2}$ (linear, 4 modes) has a symmetric stretch (IR inactive, no dipole change), an asymmetric stretch ($2349\ \mathrm{cm^{-1}}$) and a doubly degenerate bend ($667\ \mathrm{cm^{-1}}$). Only the asymmetric stretch and the bending absorption are observed, so 2.