Assertion: Sanchi Stupa survived the plunder which was witnessed by Amravati Stupa.
Reason: Perhaps Amravati was discovered before scholars understood the value of the finds and realised how critical it was to preserve things where they had been found instead of removing them from the site.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Amaravati's significance might have been overlooked in the early stages, before scholars fully grasped the importance of preserving artifacts in their original context. When Sanchi was "discovered" in 1818, three of its gateways were still intact, with the fourth lying on the spot where it had fallen, and the mound was well-preserved. However, there were suggestions to move the gateway to either Paris or London. Fortunately, several factors contributed to preserving Sanchi in its original state, allowing it to remain standing. Conversely, the mahachaitya at Amaravati suffered a different fate, becoming a mere insignificant mound, stripped of its former grandeur, as it was removed from its original location