Scientists know that dinosaurs laid eggs because ?

As far as we know, all dinosaurs reproduced by laying eggs, as do most other sauropsids (reptiles). It is very difficult to determine what species of dinosaur laid the eggs that have been discovered, because only a few dinosaur embryos have been found inside the fossil eggs.

They are the early age birds.

Explanation: Cavemen lived before us and there Neanderthol was born way before then.

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.

the answer is:

carbon dioxide