Answer:
The students set up their experiments quickly before the science fair began.
Explanation:
"Joining words" are used to connect sentences, clauses, phrases, words, etc. together in order to express a clear idea without unnecessary pauses. It also makes the thought direct, clear and concise.
"Before" as a joining word/conjunction is used in the sentence above to show how one event immediately happened after another event. So, the first event refers to the students' setting up their experiments quickly while the second event is the start of the science fair.
"They set up quickly" can be incorporated into one with "The students set up their experiments" because it talks about the same situation–the students' experiments.
