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Complete the dramatic historian's lecture notes by selecting the grammatically correct option for each gap.
The shaky foundation _________________________ the ancient empire was built eventually crumbled under the weight of terrible leadership. However, the astonishing extent _________________________ the emperors simply ignored their falling walls remains hilarious to modern scholars.
The correct answers are upon which and to which.
When a preposition is moved to the front of a relative clause, it must match the verb or noun it belongs to.
- A foundation is built upon (or on), so we use "upon which." We cannot use "that" directly after a preposition.
- The noun "extent" strongly collocates with the preposition "to" (e.g., "to a great extent"). Therefore, the correct formal relative phrase is "the extent to which."