undefined reference to xxx

Hi all,
I have joined this list newly, and I have some question this
question may be solved up to now

I wrote simple c source this is connect my postgresql database
but when I compile my source I see some errors this is listed below
gcc -g connect.c -o connec /usr/lib/libpg.a -I /usr/include/pgsql
/tmp/cc91os3S.o: In function `exit_nicely': /root/connect.c:7: undefined
reference to `PQfinish' /tmp/cc91os3S.o: In function `main':
/root/connect.c:17: undefined reference to `PQsetdbLogin'
/root/connect.c:18: undefined reference to `PQstatus' /root/connect.c:20:
undefined reference to `PQerrorMessage' collect2: ld returned 1 exit
status
How can I solve this problem?Please help me?
I look forward your message as soon as possible?
Please!