An experiment in which only one variable is changed is a(an) experiment.

a. randomized
b. natural
c. double blind
d. controlled

Hi

The correct answer is : D

Because a control experiment is one in which a hypothesis is tested by the manipulation of a single variable.

I hope that's help:)

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

for  example, forests have higher evapotranspiration than other ecosystems,  such  as grasslands, because of their deeper roots and greater leaf area. thus forests have a net moistening effect on the atmosphere and become a moisture source for downwind ecosystems.