The subjects were two groups of 10 male volunteers sealed into a mock spaceship for two simulated flights to Mars.
(source:https://www.eurekalert.org/pub_releases/2017-04/mdcf-mcf041117.php )
I have a quesiton here. Does "two groups of 10 male volunteers" mean:
(i) There were two groups of subjects, with 10 male volunteers in each group; or
(ii) There were two groups of subjects, with 10 male volunteers in total.
I have read the article, but it does not mention this infomation in other parts.
Thank you!