The life spans of rockﬁsh vary greatly by species. For instance, the colorful calico rockﬁsh (Sebastes dalli) can survive for a
little over a ______ the rougheye rockﬁsh (Sebastes aleutianus) boasts a maximum life span of about two centuries.
decade: while
decade. While
decade; while
decade, while
Choice D is the best answer. The convention being tested is punctuation between a main clause and a subordinate clause.
This choice correctly uses a comma to mark the boundary between the main clause (“the colorful…decade”) and the
subordinate clause (“while…centuries”) that provides contrasting information about the life span of rougheye rockﬁsh. 
Choice A is incorrect because a colon can’t be used in this way to join a main clause and a subordinate clause. Choice B is
incorrect because it results in a rhetorically unacceptable sentence fragment beginning with “while.” Choice C is incorrect
because a semicolon can’t be used in this way to join a main clause and a subordinate clause.