参考答案：B

解析：在所提供的四个选项中只有B是固定搭配词组，意为“在现场，当场”，如：The police were on the spot within a few minutes of hearing about the crime．获悉该项犯罪后几分钟，警察即赶到了现场。The bullet struck his head and he was killed on the spot．子弹击中了他的头部，他当场就被打死了。很显然，此题需要的意思为“当场”，所以答案应为B，其他的选项均为起迷惑作用的干扰项。