Let me make it simpler.
column A stores the exam subjects that I am going to filter. I found that
after applying the data--filter--autofilter

(with some creteria) on column A, a certain subject(say English) is
filtered. Then I try to use autofill in column B to

assign seat number to the students(001, 002, 003,..). However, I cannot fill
in the series. Can anybody help me?