A modifier question tests whether an opening describing phrase points to the right noun after its comma. Read the phrase and ask who or what it describes; if something was done, ask who or what it was done to. The main noun right after the comma must be that person or thing. Name it first, then test each choice's main noun and read on. Commas can set off extra phrases in pairs; they don't separate a subject from its verb.
Hints
- Hint 1
Ask what was installed along the bridge. The opening phrase has to describe the main noun after its comma.
- Hint 2
Once you've found what was installed, check how its noun connects to its verb. Read the completed sentence, including any commas, to check that its parts fit together.
Step-by-step
Match the noun and check the comma
Step 1Name what was installed
Installed along an aging suspension bridge to monitor structural stress says that something was put along the bridge, but it doesn't name that thing. What was installed? The phrase points to equipment used to monitor the bridge. This opening modifier, a phrase that describes a noun, must describe the main noun right after its comma, so that noun has to name the equipment.
- Step 2
Test the first nouns
Which main noun gets described as Installed along an aging suspension bridge in each version? The tiny changes version starts with changes, but vibration changes aren't equipment that can be installed. The it has been shown version puts it first, and here it names no particular thing. The system appears later after by, but the opening phrase describes the main noun right after its own comma.
- Step 3
Connect the system to its verb
The main noun in the city's BridgeWatch system is system, since the city's only says whose system it is. A system can be Installed along an aging suspension bridge, so what verb does that subject connect to? Its verb is has shown, and no single comma can split system from has shown, so the version with a comma after system fails. With no comma there, the system has shown that tiny changes in vibration patterns can alert engineers to problems before cracks appear, which completes the sentence. Choice B.