Short-spined sea urchin fossils were found directly above long spined sea urchin fossils in a layer of rock. what do the differences between the two sets of sea urchin fossils suggest about them?

a. the longer spines developed as the shorter-spined sea urchins grew older.

b. the longer-spined sea urchins existed before the shorter-spined sea urchins.

c. the shorter-spined sea urchins stretched their spines to ward off predators.

d. the shorter spines did not protect the sea urchins well enough and they died off.

Choice B, because of evolution. The longer spines weren't needed, so they essentially died off, leaving urchins with shorter spines

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!