confusing SC

workhard77

At the time of the Mexican agrarian revolution, the most radical faction, that of Zapata and his followers, proposed a return to communal ownership of ( land, to what had been a pre-Columbian form of ownership respected by the Spaniards ).

这题为什么A对？“to what had been...”在这里做什么成分？是什么用法？

princess

做a return to communal ownership of land的同位语

tongxun

解释、说明return to 什么。