How is malware detected when it has infected a target system?

Each correct answer represents a complete solution. Choose all that apply.

A $$ Users notice abnormal behavior of their systems, ranging from the sluggish response, to strange crashes, to unusual warning messages or pop$-$ups

B $$ Malware scanner programs look for signatures in program files that match known malware, or look for patterns of behavior that are suspicious. $$ It's very difficult to detect without examining the executable code of systems kernel files. $$

User files disappear, are corrupted, or become unusable, and then inexplicably they come back.