A scientist wants to test the effect of water temperature on the success of developing frog embryos. The scientist monitors developing embryos in different water temperatures and counts the number of embryos that mature into tadpoles.

Why should the scientist keep all variables except water temperature constant?

A to test multiple variables at once
B to ensure at least some embryos develop into tadpoles
C to ensure that the results only reflect changing temperature
D to protect the developing embryos

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.

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.

answer is d hope this