Which of the following is true about natural selection?
a. natural selection usually causes a species to change suddenly.
b. natural selection does not cause a species to change.
c. natural selection is the only cause of changes in a species.
d. natural selection usually causes a species to change gradually. i beg me 18 points if you

Natural selection is a one of the basic mechanisms of evolution by which species adapt to their environment in order to increase their chance of survival and reproduction. It is the change in the characteristics that lead to survival. In natural selection, genotype variations that will increase the chance of survival and reproduction of some organism are preserved and will be inherited. So, natural selection will "select" those genotypes that help survival causing gradual change of species.

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.