Some people have very good memories and can easily learn quite a long paragraph by heart. But they often forget them almost as quickly as they learn them. There are other people who can only remember things when they have said them over and over, but when they do know them they don’t forget them.The human mind is rather like a camera, but it takes photographs not only of what we see but of what we feel, hear or smell. There is much to do before the photograph is finished and ready to show to our friends. In the same way, there is much work to be done before we can make a picture remain forever in the mind. There is much work to be done （） we remember something forever.

A. after
B. when
C. unless
D. before