SAT Question 1b97cce9: Boundaries answer and explanation

English - Boundaries - Period Between Sentences
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Hegra is an archaeological site in present-day Saudi Arabia and was the second largest city of the Nabataean Kingdom (fourth century BCE to first century CE). Archaeologist Laila Nehmé recently traveled to Hegra to study its ancient ______ into the rocky outcrops of a vast desert, these burial chambers seem to blend seamlessly with nature.
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Difficulty:Hard
Which choice completes the text so that it conforms to the conventions of Standard English?