In my driving lesson, I learned a traffic rule that at no time _____ parked in a manner which will block emergency service.
有人形容20世纪初的巴尔干半岛就像一个存满了炸药的火药桶，只要有一点火星，就足以引爆整个欧洲。下列哪一事件起了这种“火星”的作用（ ）
