What is a variable?
a. a statement that explains a phenomena and can be tested.
b. an educated guess used in an experiment.
c. something that must be kept constant in an experiment.
d. a value or characteristic that can take different values.

The Answer is D.

Explanation:

This is because a variable can be changed and isn't dependent on the problem.

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.