Why is a longitudinal study better than a cross-sectional study for examining how many years cancer survivors stay in remission?

A cross-sectional study examines all of the participants at one time. So for a cross-sectional study of cancer survivors, we would gather some sample of former patients and find out how long they have been in remission.

The problem with that cross-sectional design is we would be missing the people whose cancer had returned and caused them to die before our experiment started; we would have introduced a problem called "survivor bias" by only counting people who were STILL in remission and still alive.

A better experiment will use a longitudinal design of enrolling people whose cancer has just gone into remission, and following them to see how long they stay healthy.

C) to ensure each new cell has an exact copy of the dna