In a randomized, controlled, double-blind study: select one:

a. experimental subjects are blind-folded when given the experimental treatment.
b. individuals will be assigned to an experimental or control group depending on whether they took part in a pilot study.
c. all experimental variables are held constant.
d. neither the experimenter nor the subject know whether the subject is in a control group or an experimental group.
e. all of the above.

d

Explanation:

In a randomized, controlled, double-blind study neither the experimenter nor the subject know whether the subject is in a control group or an experimental group.

Randomized double-blind placebo control (RDBPC) trials are considered the epidemiological research of  "gold standard." The best analogy is placebo management, which requires blindness of subjects, researchers and test workers. Trial's advantage over an observational study is its the ability to convey causality.

Hurricane Katrina would not have caused early onset of spring. The damaging winds/rains of the hurricane would have caused a change in marine ecosystems and habitats since the heavy winds/rain and storm surge eroded beaches, took down trees, and flooded out areas. As a result, animal migration also changed since some habitats that animals would normally migrate to were destroyed and no longer existed. The early onset of spring (aka. seasonal change) is not driven by individual weather systems.