Which of the following identifies what happens by day 21 of cattle gestation? the heart begins to beat
the teeth begin to form
the placenta is formed
the calf is covered with fur

frogs. the lower the ph the higher the acid content. frogs have the highest tolerance in this scenario.

imagine a line passing through the center of earth that goes through both the north pole and the south pole. this imaginary line is called an axis. earth spins around its axis, just as a top spins around its spindle. this spinning movement is called earth's rotation. at the same time that the earth spins on its axis, it also orbits, or revolves around the sun. this movement is called revolution.