Ideas within a sentence, and sentences as well, can be connected in a text through several means. One of them is punctuation and the other, the use of linking words, like but, for, and, of, etc. Yet a third way is through the use of connecting words, sometimes in the ending clause that help to give meaning to two sentences that would initially seem unconnected. In this case, the first sentence talks about the best time of a person´s life, the night of Sherry´s party. This word of night, establishes the time at which the event took place. The second sentence, which has no noun, no pronouns, just a descriptor like worst, it would seem like the sentences have absolutely no connection. Except that the second sentence ends with the words, next day, which again not only establishes connection through time, but also chronologically organizes the two ideas. The first idea being the best time during a night, the next idea being the worst time the following day. This is why the correct answer is A.