Answer to Question 1
D
Answer to Question 2
It is possible to demonstrate a relationship between two variables by simply observing them. However, simple observation cannot establish the direction of the relationship. For example, you may observe that two variables change together consistently: as variable A increases, variable B also increases. However, you cannot determine whether changes in A are causing changes in B, or whether changes in B are causing changes in A. To avoid this problem in an experiment, the researcher takes charge and manipulates one of the variables. For example, the researcher increases the value of variable A and then observes variable B to see if it is affected by the increase. If B also increases, then the researcher can conclude that changing variable A has caused changes in variable B.
Testing the reducing atmosphere hypothesis for the origin of life - the Miller and Urey experiment.
The experiment of Plaxton and his colleagues demonstrated that plants can alter their metabolism in
Experiment on newt embryos that led to the discovery of Spemann's organizer.
The Meselson-Stahl Experiment
EXAMPLE OF A RANDOMIZED CONTROLLED TRIAL
How to rearrange formulas to isolate for a variable (Part 2)