When the UFO(landed/was landing),the dog (slept/was sleeping

When the UFO(landed/was landing),the dog (slept/was sleeping)
另有一题：When the reporter(arrived/was arriving),everybody(shouted/was shouting)这两题标准答案：第一题landed,was sleeping；第二题arrived,shouted.但我不明白为什么句式基本相同的这两题答案不同（第一题一个用一般过去,一个过去进行；第二题两个都是一般过去,为什么?）还有,这类题应该怎么判断所用时态?