I have a text file `test.txt` with something like the following:

 foo
 configure
 top
 start
 fun
 bar
 hello

I want to comment out line numbers 2, 5 and 7, so that the output would look like:

 foo
 #configure
 top
 start
 #fun
 bar
 #hello

When I use `sed -i "2,7 {s/^/#/}" test.txt` lines 2, 3, 4, 5, 6 and 7 get commented. How to ensure only specific line numbers are commented?
Appreciate any help.