Proof Writing & Problem Solving · Easy problem

How many of the following are correct principles of proof writing?

(A) Checking a "for every $n$" claim on several examples proves it.

(B) A single counterexample disproves a "for every $n$" claim.

(C) Proving the contrapositive "if not $Q$ then not $P$" proves "if $P$ then $Q$".

(D) Proving the converse "if $Q$ then $P$" proves "if $P$ then $Q$".