A data engineer is curating data in the silver layer of a hospital management data warehouse system. The data engineer is trying to aggregate hospital billing data from a table patient_billing to generate a daily revenue fact table daily_revenue.

Assume this as a sample of dataframe billing_df:

billing_id	patient_id	department	billing_date	amount_billed	quantity
401	p001	Cardiology	2024-03-01	1500	1
402	p002	Radiology	2024-03-02	3000	1
403	p001	Cardiology	2024-03-01	6500	1
404	p003	Radiology	2024-03-03	500	1

Which code snippet aggregates the amount billed per day with the unique invoices from a Dataframe billing_df? (Options were provided separately.)