Answer:
C. The marching band—which has been practicing for weeks—won a blue ribbon at the competition
Explanation:
This is the correct placement of the dashes.
This is because the dash should only highlight the main object of the sentence which is
"which has been practicing for weeks" to make it clearer to the reader.
The main object of the sentence asks the question WHAT?
The main subject of the sentence answers the question WHO?
WHAT has the marching band been doing?
They have been practising for weeks
WHO has been practising for weeks?
The marching band.
