Exchange of trace evidence occurs, When a person comes in contact with other person . This principle is called?: (A) Locard's principle, (B) Quetelet's rule, (C) Petty's principle, (D) None of the above
Answer is A. Locard's principle of Exchange 

It States that " When two objects come into contact with each other, there is always transfer of some material between them".
This is helpful in crime investigations, especially in sexual offences.