a question of sed

hi
I got a question when I use sed, please refer to the log:
*******************************************************************beginning
of log
root@roger-T540p-Ubuntu:/home/roger/work/shell# cat emp.txt
100 Jason Smith
200 John Doe
what happened?!
300 Sanjay Gupta
400 Ashok Sharma
500 roger chen

root@roger-T540p-Ubuntu:/home/roger/work/shell# sed -n -i '3d' emp.txt
root@roger-T540p-Ubuntu:/home/roger/work/shell# cat emp.txt
root@roger-T540p-Ubuntu:/home/roger/work/shell#
*******************************************************************end of
log

I just want to delete the third line " what happened?!", and after the
command, all lines were deleted.

did it suppose to be ?

thank you for your time!


-- 
Best Regards,
Roger Chen