Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) At least two hospitals in Northern California evacuated patients as deadly wildfires spread across at least 57,000 acres, destroying 1,500 structures, officials said Monday. Roughly 130 patients were evacuated from Kaiser Permanente's Santa Rosa medical center on Monday morning. They were transported to Kaiser Permanente's San Rafael center and other local hospitals and evacuation sites, according to a statement by the health system. Another 70 patients were evacuated from Sutter Santa Rosa Regional Hospital to other area hospitals by 8:45 a.m. Monday, according to a statement from Sutter Health. "The hospital has no known structural damage, but remains closed at this time," the statement said.Two hospitals have evacuated patients as wildfires spread across Northern CaliforniaWildfires have hit at least 57,000 acres, destroying hundreds of structures 
 Question:What caused the simultaneous fires is unknown, according to _ Fire Chief Barry Biermann.

Northern California