In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If rain is water, water is road, road is cloud, cloud is sky, sky is sea and sea is path, where do aeroplanes fly ?
The aeroplanes fly in the 'sky' and the 'sky' is called 'sea'.
So, the aeroplanes fly in the 'sea.