(1) A prosperity which / that had never been seen before app

只要看which or that 所指代的先行词在句子中做什么成分。
A prosperity which / that had never been seen before appears in the countryside。（which / that）指代的内容是A prosperity在定语从句A prosperity which / that had never been seen before中作主语。即
A prosperity had never been seen before。
a car （that） can travel 150 miles an hour。（作主语）
The car that he drives can travel 150 miles an hour。
{The car can travel 150 miles an hour。这是主句部分。}
拿着the car 往后面定语从句部分（he drives）去放,应放在 he drives 后。。也就是做drive 的宾语。