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