A dog has a piece(片) of meat in his mouth. He sees himself(他自己) in the water. He thinks it is another dog. That dog has a piece of meat in his mouth，too. He wants to get(得到) his meat. Then he can have two pieces. He opens his mouth to bark(吠叫) and his meat goes down into(在……里面) the water.

三个共点力大小分别是F₁、F₂、F₃，关于它们合力F的大小，下列说法中正确的是（   ）