Iron is concentrated in earth’s core because of the a. movement of materials due to heating in earth’s protoplanetary phase. b. sequence of accretion of material during earth’s formation. c. radioactive decay of isotopes in earth’s core to produce iron. d. penetration of iron meteorites to earth’s core when earth was smaller than it is now.

The answer is A. During the formation of earth, the friction as elements collided into a mass caused heating, so earth was them constituted majorly of molten lava. The heavier elements like nickel and iron were submerged to the core. This is because, since they are among the heaviest elements, the effect of gravity on them was the greatest.

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!