The examiner has selected
answer (a) however:
Saturation vapour pressure
is lower over ice than it is over water: i.e. it takes less molecules of water
vapour to saturate the air over ice than it does over super-cooled liquid
water. The dew point is the temperature at which the air is
saturated and any further cooling results in molecules of water vapour
condensing to form liquid water. The frost point is the temperature
at which the air is saturated and any further cooling results in molecules of
water vapour sublimating; i.e. they miss the liquid state and go straight from
vapour to ice. If ice is already present and the temperature at
which the air becomes saturated is below 0ºC then sublimation will occur at a
slightly higher temperature than condensation - the frost point temperature is
higher than the dew point temperature. On this basis answer (c)
could be correct and the question could be appealed.