Difference between prior to and before

[Romina]

Test No. incompl/advan-86 "GMAT Verbal Quiz (9)", question 1

Remember that this happened first and was ......... to the episode that happened afterwards.

(a) before
(b) since
(c) prior
(d) previous

Test No. incompl/advan-86 "GMAT Verbal Quiz (9)", answer 1

Remember that this happened first and was prior to the episode that happened afterwards.

Correct answer: (c) prior

Your answer was: incorrect
Remember that this happened first and was before to the episode that happened afterwards.
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Why prior instead before?

[Alan]

As you can see, the expression is prior to. You can't use before with to.
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