The diagram below shows the layers in a rock having a brachiopod: a rock column labeled rock is shown. the column has three layers. the bottommost layer of the rock has two identical organisms marked as, brachiopod, in the key and one shaded circle, the middle layer has a two squares, the top layer has one star shape. in the key the shaded circle is marked as x, the square is marked as y and the star is marked as z which statement about the rock fossils is true? organism x existed for a short period of time. organism y became extinct before organism x. organism x and organism y existed at the same time. organism z became extinct much before organism x.

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Explanation:

i took the test and got it right.

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!