The correct answer is C because billing occurs either monthly or when the threshold is reached, whichever comes first. With an average spending of $50 per day and 18 days passed, the$1000 threshold would be reached in 2 days, triggering the billing. Options A and D are incorrect because the threshold would be hit before 12 days pass. Option B is incorrect as billing is not necessarily on the first day of the next month. Option D is incorrect because there is no fixed billing date like the thirtieth day of the month. Refer to GCP documentation on Cloud Storage Billing for more details.