Now that VerbalBot has raked this up yet again, let’s revisit it. The OA is supposed to be C as per the elaborate discussions done hitherto
C) their populations have declined dramatically since the 1970s, a period in which there were fewer bald eagles competing with them for food and nesting sites.
The preposition since indicates (erroneously IMO) that the declines have been happening since 1970 until now or for that matter may even continue further. However the following part after the timeline
...
C) their populations have declined dramatically since the 1970s, a period in which there were fewer bald eagles competing with them for food and nesting sites.
The preposition since indicates (erroneously IMO) that the declines have been happening since 1970 until now or for that matter may even continue further. However the following part after the timeline
...
