The classic simplistic approach - the CAA are testing your syllabus not your knowledge. That question takes no account of the fact that the dew point changes with height!
The question does say APPROXIMATE height.
And for those of you who are confused by the Whirleys....
WhirlyBIRD = Whirly
WhirlyGIG = Whirls
Someday - at the Gatbash, even - you will see us together, and panic because you think you're seeing double. But we're TWO different people, honest!