in a crystal report version 9 report, i got several detail sections.:
details a
details b
details c
etc.

details a is only one short line info but details c has big size data.

details a starts from anywhere in the page , details c starts from start of next page.  maybe reason is to fit data. So, there is empty lines between details a and details c. How can i make details c follow right after details a?. no matter where details a starts or ends , i want details c follow right after.

it is now:
details a : somename1
at start of next page:
details c:  data data ...
details a : somename2
at start of next page:
details c:  data data ...

But, what i want is:
details a: somename1
details c: data data ...
details a: somename2
details c: data data ...
details a: somename3
details c: data data ...
etc.

Odds are very good, actually, that you have "Keep Together" checked somewhere.  Look through your Section Expert and Group Expert, and try toggling any "Keep Together" options until you get the look you want.  If your Details C contains a large text field (as I suspect it does), you will also want to go into the format properties for that field and uncheck "Keep Together."