### **Official Solution & Concept Analysis (GATE BT 2022 Q.1)**
**1. Governing Concept**:
- **Topic**: Verbal Ability & Homophones / Contextual Word Choice
- Sentence structure: *"You should _________ when to say _________."*
- First blank: Requires a verb expressing awareness, comprehension, or discernment $\rightarrow$ **"know"**.
- Second blank: Requires a noun/refusal expressing negation $\rightarrow$ **"no"**.
- Sentence: "You should **know** when to say **no**."
**2. Option Analysis**:
- **Option (A) [no / no] is INCORRECT**: "no" is not an action verb for the modal "should".
- **Option (B) [no / know] is INCORRECT**: Inverted sequence.
- **Option (C) [know / know] is INCORRECT**: Second blank cannot be the verb "know".
- **Option (D) [know / no] is CORRECT**: Grammatically and semantically idiomatic English proverb.
**3. Final Answer**:
- **Correct Choice**: **D**
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