An editor wants the marked sentence to preserve the passage's intended time relationship. Paragraph 1: A greenhouse alarm sounds when the temperature drops below a safe level. The overnight log records both the temperature and the heater response. Paragraph 2: The writer describes the earlier condition that triggered the alarm. Marked sentence: "The temperature had fell below the warning threshold before the heater restarted." The plants showed no signs of damage the next morning. Which option uses the required past participle in the complete sentence?