The duty of our teacher is to give instructions; … is to obey.

Analyze the question The sentence is: "The duty of our teacher is to give instructions; __ is to obey." We need the word that fits the blank and refers back to "the duty" of the students.

Explain the concept: possessive pronouns

• A possessive adjective (my, our, your, their) comes before a noun: our duty.
• A possessive pronoun (mine, ours, yours, theirs) stands alone and replaces "possessive + noun": ours = our duty.
• Possessive pronouns never take an apostrophe: ours, yours, theirs, hers, its.

Step-by-step solution

1. After the semicolon, the blank is the subject of "is to obey". No noun follows it, so it must stand alone.
2. "Ours" replaces "our duty", giving: "The duty of our teacher is to give instructions; ours is to obey."
3. Check the other options: • B. our: a possessive adjective that needs a noun after it ("our is to obey" is wrong). • C. ours' and D. our's: apostrophes are never used with possessive pronouns, so both are incorrect.

Answer: A. ours

Tip: If a possessive word stands alone with no noun after it, use the "-s" form (ours, yours, theirs) and never add an apostrophe. The only common exception is it's, which means "it is" or "it has", while the possessive form is its.