Which of the following regular expressions turns this input stream into the following output stream?

exams 101-500 101-500 exam 1 Comment

Given the following input stream:

txt1.txt atxt.txt txtB.txt

Which of the following regular expressions turns this input stream into the following output stream?

txt1.bak.txt atxt.bak.txt txtB.bak.txt
A . s/^.txt/.bak/
B . s/txt/bak.txt/
C . s/txt$/bak.txt/
D . s/^txt$/.bak^/
E . s/[.txt]/.bak$1/

Answer: C