SQL & PL/SQL

Hi,
I am using oracle8i. i tried to write a procedure for a banking project. there are two options. one is credit another is debit. if we enter 'credit' i should get the prompt(old:) credit, otherwise it will be 'debit'. it write the code as follows:
table name is bank1;
declare
tcr bank1%type;
tdr bank1%type;
opt bank1%type;
begin
select tcr, tdr, opt into tcr, tdr, opt from bank1;
opt:=&opt;
if(opt='credit')
tcr:=&tcr;
tcb:=tcb+tcr;
elsif(opt='debit')
tdr:=&tdr
tcb:=tcb-tdr;
end;
update bank1 set tcr=...........;
end;
here i encounter a problem: even if i give the 'if..elsif..'condition, the prompt ask both tcr and tdr. why? i need either one of that. how do i do?
Thank you.