Final answer:
A controlled experiment only tests one variable at a time and includes a control group that does not receive the experimental treatment. Controlled experiments are used to assess the effect of one independent variable on a dependent variable and can be done on both non-living and living things, including humans.
Step-by-step explanation:
The correct statement about controlled experiments is that a controlled experiment only tests one variable at a time. This means that while all other conditions are kept constant, only the independent variable is altered to assess its effect on the dependent variable. It's important to have a control group that does not receive the experimental treatment, acting as a benchmark to measure the effects of the independent variable. For example, in an experiment to test whether a nutrient affects algal growth, you would have ponds with the nutrient and ponds without it (as controls) to compare the differences in algal growth.
Regarding hypotheses, it's essential to note that they must be testable and falsifiable through scientific experiments, but they need not be valid in all situations in the universe. Additionally, while larger sample sizes can provide more statistical power, experiments can indeed be conducted on humans, provided ethical guidelines are strictly followed.
Experimental controls, such as keeping all conditions constant except the independent variable, allow for meaningful comparisons between test groups and control groups, ensuring that observed effects are due to the variable being tested and not other factors.