The correct answer is A.
An Adverb Clause, also known as Adverbial Clause has the function of an adverb, and, in this case, it answers the question how, and talks about a possible situation and consequences: If you don't buy the tickets in advance, <em>you may not be able to get them</em>. The second sentence is a possible situation in the case of a delay to buy the tickets.
It means that it is impossible to get something for nothing, or the idea that even if something seems free, there is always a cost, no matter how indirect or hidden the cost is.
This is incorrect.
It is not always true because there might be other reasons for your unhappiness that are not related to hunger.
