一段英语短文的中心句的问题-----------高手帮忙

一段英语短文的中心句的问题-----------高手帮忙
There is an old saying in English:“Laughter is the best medicine”.Until recently,few people took the saying very seriously.Now,however,doctors have begun to investigate laughter and the effects it has on the human body.They have found evidence that laughter really can improve people’s health.
第1：有人说引号里面的是中心句“Laughter is the best medicine.”
第2：有人说however后面的是中心句“doctors have begun to investigate laughter and the effects it has on the human body”
第3：有人说found evidence后面的同位语从句“laughter really can improve people’s health”是中心句.
晕了,都说的很有道理,到底哪段是中心句,说明道理.