In a study of physical endurance, researchers observed signincant increases in the heart rates and breathing rates of participants immediately after they engaged in strenuous exercise
Which statement best explains the increase in the heart rate and the breathing rated during exercise?
The water concentration in the blood increases.
Body cells require increased oxygen as energy is expended.
Muscle cells increase in temperature and require fuld to reduce the temperature.
An increase in muscle activity causes increases in glucose levels in red blood cells.

answer; /// i believe that the correct answer is ( monophasic ); ethinyl estradiol- ethynodiol diacetate is a monophasic oral contraceptive agent; ///

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!

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.