The similarity between the stars' motions initially seemed to support the conclusion that they formed a binary _____ showed that the stars were hundreds of light-years apart, astronomers ultimately rejected that conclusion.
Which choice completes the text so that it conforms to the conventions of Standard English?
For difficult boundary questions, classify complete sentence units rather than judging only the words next to the blank. A unit that begins with a subordinating conjunction such as “because” can still be complete if that dependent clause is followed by its own independent clause; then determine what punctuation correctly connects that entire unit to the preceding clause.
Hints
Map the main clauses
Identify the subject and main verb in the material before the blank and in the material ending with “rejected that conclusion.”
Evaluate the full second unit
Do not decide that everything beginning with “because” is necessarily a fragment. Determine whether a main clause follows the introductory dependent clause.
Step-by-step Explanation
Identify the first sentence unit
“The similarity between the stars' motions initially seemed to support the conclusion that they formed a binary system” has a subject, “similarity,” and a main verb, “seemed,” so it is an independent clause.
Analyze the entire second unit
The clause “because later measurements showed that the stars were hundreds of light-years apart” is dependent, but it is followed by the independent clause “astronomers ultimately rejected that conclusion.” Together, these clauses form a complete complex sentence.
Choose the valid boundary
The completed text contains two complete sentence units. A semicolon can separate them, even though the second unit begins with a dependent clause: “system; because later measurements showed that they were hundreds of light-years apart, astronomers ultimately rejected that conclusion.”