In earth's history there has(have) been major extinction event(s) leading to the loss of 50% or more of the species present, not counting the current mass extinction.

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.

What best explains the inability for life to exist in earth early atmosphere

Why should organisms reproduce more offspring than will survive? select all that apply.