Which of the Following Is an Example of Secondary Picketing

Which of the following is an example of secondary picketing?

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