A scientist performs an experiment to see how plants respond to acid rain. He places the water plants in water of varying pH measurements. The pH levels are 2.5, 6.0, 7.0, and 9.0. Each day, he measures the health of the plants by counting the number of leaves on the plant. The plants used in this experiment typically grow in water at a neutral pH. Why was only one variable tested in this experiment?

The scientist tested one variable to determine what caused the number of leaves to change.
It just happened that way; you can test as many variables as you want.
The scientist ran out of materials so he tested only one variable.
The scientist tested two variables.

eukarya because is is like alche and b is bacteria and d is animals

explanation:

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!

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.