Official Explanation -
Where formal logic is concerned, even one word can make a big difference.
The operative word in the stimulus is most. If in most partsthat is, in well beyond 50%of this country, its either easy to grow cacti (meaning that those parts are not humid) or easy to raise orange trees (meaning that those parts are not cold), then most of the country has to be either not humid, or not cold, or both. For that reason (A) has to be false. If (A) were trueif half of the country were indeed both humid and coldthen at maximum, only the other 50% of the country could grow cacti or raise oranges with the ease that the stimulus describes. And 50% doesnt qualify as most.
Meanwhile, three of the wrong choices could be true, and one of them must be:
(B) The terms of the argument are humid and cold. Hot is never mentioned, so its eminently possible for the entire country (let alone most of it) to be hot.
(C) Has to be true. In fact, the some parts of which (C) speaks have to comprise the majoritythe parts where its easy to grow cacti or raise oranges or both.
(D) This statement could be true. The stimulus only requires that it is easy to grow cacti or orange trees in most parts of the country. It could be impossible to grow cacti in the country as long as it is easy to grow orange trees in most parts of the country.
(E) Could be true. If true, then most of the country must not be cold so as to make it easy to raise orange trees there. Thats consistent with the stimulus.
Always a delight to solve such tricky formal logic questions.
Very easy to get wrong when in a hurry.As usual generis hits the nail on its head and then some. +1 for that.