The stimulus tells us that "in most parts of a certain country, it is either easy to grow cacti or easy to raise orange trees." So in more than half of this country, it is easy to either grow cacti or raise oranges trees. The stimulus also tells us that it is difficult to grow cacti in humid climate and it is difficult to raise orange trees in a cold climate.
As such, it must follow that more than half of this country is either not humid or not cold (possibly neither humid nor cold).
This is where the "half" comes from. We are looking for what cannot be true. If half of the country is both humid and cold, it would be impossible for us to grow cacti or raise oranges in more than half of that country.
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