Not all memories  are sweet. Some people spend all their lives trying to forget bad experiences. Wars and traffic accidents can leave people with terrible physical and mental _ Often they appear again in dreams. Now American researchers think they are close to developing a pill , which will help people forget bad memories. The pill is designed to be taken immediately after a frightening experience. They hope it might reduce , or possibly erase  the effect of sad memories. In November, some scientists tested a drug  on people in the US and France. The drug stops the body producing chemicals  that fix memories in the brain. So far the research has suggested that only the mental effects of memories may be reduced, not that the memories are erased. The research has caused plenty of argument. Some think it is a bad idea, while others support it. Supporters say it could lead to pills that prevent or treat soldiers' troubling memories after war. They say that there are many people who suffer from terrible memories. "Some memories can destroy  people's lives. They come back to you when you don't want to have them in a dream. They usually come with very sad feeling." said Roger Pitman, a scientist of psychiatry at Harvard Medical School. "This could relieve  a lot of that suffering." But those who are against the research say that it is very dangerous to change memories because memories give us our identity . They also help us all get away from the mistakes of the past. "All of us can think of bad events in our lives that were terrible at the time but make us who we are. I'm not sure we want to erase those memories," said Rebecca Dresser. The passage is mainly about   _  .
a new medical invention a new research on memories a way of erasing sad memories an argument about the research on the pill