A joining question tests how you connect sentence parts when every choice includes the same joining word. Split before the joining word, then look for a subject and a verb on each side. Ask whether each side can stand alone. A comma pairs with a joining word between full sentences; a semicolon separates them without one, and a colon introduces an explanation or example. If the parts share one subject, the joining word usually needs no comma.
Hints
- Hint 1
Split the sentence at the blank, and read each side for a subject and a verb.
- Hint 2
Check who devised the corrective optics. Does that part have its own subject, or borrow the subject from earlier?
- Hint 3
After testing both sides, check the punctuation and joining word together. The mark has to suit the way that word connects the parts.
Step-by-step
Split and test the join
Step 1Test the mirror's part
Split before so, the joining word in every choice, and test the mirror's part first. We're deciding how to connect it to the engineers' work, so the sentence structure matters more than a pause. In primary mirror was found, primary mirror is the subject and was found is the main verb, which tells what happened to it. Adding flawed completes the thought, so the first part can stand alone.
- Step 2
Test the engineers' part
Does the engineers' part have its own subject and main verb? engineers devised an ingenious set of corrective optics gives us both: engineers and devised. The words that restored the telescope's full potential describe the optics rather than making this part depend on the mirror's sentence. This means the second part can stand alone too. So means as a result and links the mirror's flaw to the engineers' response.
- Step 3
Pair the joining word with its mark
Which mark works with so between two full sentences? A comma and a joining word work as a pair, so so needs a comma immediately before it here. The version with no punctuation runs the sentences together, while the semicolon version uses a mark that joins full sentences on its own rather than working with so. A colon introduces an explanation or example rather than pairing with this joining word, so flawed, so gives us the needed join. Choice D.