Answer to Question 1
Even though experimental designs are considered to be among the strongest of research designs, they are not without shortcomings. Many things can go wrong. First, you may find that your groups have differential dropout, thereby confounding the program with the outcome. This in effect diminishes the value of random assignment, because the initially probabilistically equivalent groups become less equivalent due to differential dropout. You might also be challenged on ethical grounds. After all, to use this approach you typically have to deny the program to some people who might be equally deserving of it as others. You could meet resistance from the staff members in your study who would like some of their favorite people to get the program. Alternatively, your design may call for assigning a certain group of people to a harmful program, which may not be plausible to implement on ethical grounds. The bottom line here is that experimental designs are difficult to carry out in many real-world contexts, and because an experiment is often an intrusion, you are setting up an artificial situation so that you can assess your causal relationship with high internal validity. As a result, you may be limiting the degree to which you can generalize your results to real contexts where you haven't set up an experiment. That is, you may have reduced your external validity to achieve greater internal validity.
Answer to Question 2
C
Summary of greenhouse experimental design and results
Research Methods: Advantages and Limitations
Design Data for Various Packings
Figure of a Woman with a Drawing Showing Evidence of Original Painting and Outlining Design Scheme
Suppose that we have a sample space with five equally likely experimental outcomes: E1, E2, E3, ...
Directional artificial selection favoring the AdhF allele in experimental Drosophila populations.