Which of the Following Is an Example of Secondary Picketing

Which of the following is an example of secondary picketing?

A) employees of a factory picketing in front of the factory's biggest supplier
B) public service workers picketing on Parliament Hill
C) university faculty picketing in front of various university buildings as part of a strike action
D) striking employees of a large hotel chain picketing at other hotels in the same chain
E) anti- poverty activists protesting at a provincial legislature