  We use inelastic neutron scattering to study the fate of the two spin
resonance modes in underdoped superconducting NaFe$_{1-x}$Co$_x$As ($x=0.0175$)
under applied magnetic fields. While an applied in-plane magnetic field of
$B=12$ T only modestly suppresses superconductivity and enhances static
antiferromagnetic order, the two spin resonance modes display disparate
responses. The spin resonance mode at higher energy is mildly suppressed,
consistent with the field effect in other unconventional superconductors. The
spin resonance mode at lower energy, on the other hand, is almost completely
suppressed. Such dramatically different responses to applied magnetic field
indicate distinct origins of the two spin resonance modes, resulting from the
strongly orbital-selective nature of spin excitations and Cooper-pairing in
iron-based superconductors.
