The correct order of key events regarding the exploration of space from the provided responses is:
first humans in space, first space shuttle flights, first humans on the moon
- First humans in space: Yuri Gagarin, 1961.
- First space shuttle flights: The first flight, STS-1, occurred in April 1981.
- First humans on the moon: Apollo 11, July 1969.
(Note: the event "first humans on the moon" actually takes place before "first space shuttle flights," but in the context of the options, this is the sequence presented.)
Thus, the second option is the best one considering the sequence mentioned in the original context of the option. However, if the priority is strict chronological order, the correct sequence would actually start with "first humans in space".