I think that the missing bit of the puzzle here is that the Dew Point also reduces by around 0.5C/1000'. This is where the 2.5C/1000' reduction in difference between temp and dewpoint comes from (3C drop in temp, 0.5C drop in dew point results in 2.5C reduction in difference between temp and dew point).
My reaction is that this is one of the CAA/JAA questions where you have to pick the "least wrong" one - 2400' is closer to 2000' than any other answer.
OC619