Answer:
The students didn't prepare for the test, for they didn't realize how important the test was.
Step-by-step explanation:
The given two sentences are independent sentences. In order to link them into a compound sentence by displaying a reason, a coordinating conjunction must be employed so that each sentence carries equal significance. The coordinating conjunctions 'so' and 'for' out of 'FANBOYS' are primarily used to denote reasons where 'so' highlights the effect while 'for' emphasizes the 'reason or cause.' Thus, 'for' has been used to join the two sentences into a compound sentence where the former('The students didn't prepare') denotes the effect, and the latter displays the cause(for they didn't realize the importance of the test').