Answer:
The sentence that has a clue that two things are being contrasted is sentence 1 "You may never have thought about it, but life on a farm is very different from life in the city."
Step-by-step explanation:
The first sentence of the excerpt presents the two elements that will be further described, some clue words that show the reader that they will see a comparison of two things are "very different from" since before and after this expression we can see the two elements to be compared which are life on a farm and life in the city.