17. According to scientific evidence, which event would have been the first to occur during the formation of the Solar Systern? A collapsing cloud of gas and dust formed accretion disk around the young Sun.
A Mars-sized object collided with Earth, which led to the formation of the Moon.
A period of heavy bombardment of meteorites merged together to form the inner planets.
A cooling period occurred, which allowed for rocks on the new planets to harden.

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.

only about ten percent of the energy at one trophic level is passed on to organisms at the next trophic level. the remainder of the energy is used in life processes or lost as heat.