What goes in the gap here?
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ANSWER: rule
'This rule' refers BACK to the previous sentence about the width and height of the houses.
'This rule' summarises the whole sentence in a single noun.
This referencing system (using nouns to summarise sentences) has various names like:
Anyway, I'm sure you get the idea.
These nouns are really useful for avoiding repetition in your writing and speaking - check out the full list in the lesson below (go to point 3 and the slides).