How to put a comment at the begining of the sentence

Shell Programming and Scripting
1

I have a file shows as below. I would like to put # before CCCC. so how to do in Solaris. Here sed doesnot support -i

[StartUpTasks]

[RunTimeTasks]
AAAA
BBBB
CCCCC

DDDDD

EEEEE
FFFFFF

2 
awk '/^CCCC/{$0="#"$0}1' file

Regards