marine should be option A as it serves as a control group (baseline to compare results to). the salt will have no impact, similar to what a placebo would do. it wouldn’t be option B as having some sort of light isn’t providing a baseline as the variable of light is still being manipulated. option C and D would be incorrect as they are alterations to the experiment thus various experimental groups.
girlmeetsvce i replied to your email but ill provide my answer here too. in essence, in the context of this experiment, they're testing the effect of caffeine through the correlation of gender. gender isn’t being investigated explicitly, rather the effects of caffeine per gender. i would put this into a context of a between subject design where the same participants are used in both groups— you can change caffeine consumption but you cannot change gender (in an experimental context).
