One has to be ..... guard when downloading files from the Internet if he or she wants to avoid getting a computer virus. (*) in (*) on (*) out (*) under
This is a companion discussion topic for the original entry at https://english.best/questions/17326,one-has-to-be-___-guard-when-downloading-files-from-the-internet-if-he-or-she-wants-to-avoid-getting-a-computer-virus/