Which cells become activated only if the antigen appears in the body at a later date?

Which description provides the best objective summary of the article in the passage?

Which kind of worm has a closed circulatory system? a planarian b fluke c pinworm d an earthworm

4. multiple crosses were made between true-breeding lines of black and yellow labrador retrievers. all the f1 progeny were black. when these progeny were intercrossed, they produced an f2 consisting of 91 black, 39 yellow and 30 chocolate. a. propose an explanation for the inheritance of coat color in labradors. b. propose a biochemical pathway for coat color determination and indicate how the relevant genes control coat coloration.