When a fire ______at the National Exhibition in London, at least ten priceless paintings were completely destroyed.
【正确答案】 B
【答案解析】break out意为“(战争、疾病)的突然发生”，符合句意。break off意为“中断，绝交”；break down意为“抛锚，出故障”；break up“打碎，分解”。