Acactus with long spines protects itself from animals better than a cactus with short spines. according to natural selection,
short-spined cacti will survive longer and reproduce more often than long-spined cacti
long-spined cacti will survive longer and reproduce more often than short-spined cacti
over time, the population will become entirely made of short-spined cacti
over time, cacti with long spines will completely die out

Which is an advantage of having memory cells when a pathogen is encountered for a second time. a) the memory cells are what proliferate into clones of cells in response to the binding of an antigen. b) memory cells are vital to the primary immune response in that they immediately recognize pathogens even at the first encounter. c) memory cells are essentially effector cells that are short lived and attack antigens even without having a receptor specific to that antigen. d) it ensures that more lymphocytes with a receptor specific to a particular antigen will be present than in a host that had never encountered that pathogen.