When the temperature of a gaseous sample is increased from 298 K to 398 K, the observed microwave rotational spectrum shows broader and less intense individual lines. Which statement most plausibly accounts for this observation?
(a) The rotational constant B increases substantially with temperature owing to vibrational expansion of the bond, so previously distinct lines shift and overlap, broadening the spectrum.
(b) A higher temperature populates many additional rotational levels, distributing absorbed intensity over more transitions.
(c) Doppler broadening rises sharply at higher temperature, smearing each rotational line and lowering its peak height.
(d) Thermal energy elevates many molecules to excited electronic states in which rotational transitions are disallowed, causing the microwave lines to fade and broaden.