Solution
The first sentence states that unemployment rate has dropped down. The second contrasts it slightly by stating it may be temporary.
Thus, to connect the statements, we need a contrasting word.
But - It fits perfectly by presenting a slight contrast.
Likewise - Makes no sense here. "Likewise" is used to present another, similar case of what was previously said.
Though - Using "though" implies a contrast, but it usually works better as a conjunction within a single sentence or at the beginning or end of a sentence. It does not fit as seamlessly between the two statements in this context. The sentence would read awkwardly.
Still - "Still" implies contrast but in a way different from "but." "Still" brings out the positive part of a bad thing, for example - "He is bad at studying, still he tries." So it won't connect the two sentences.