Select the correct option:
Hardly had the rover activated ___ it began sending diagnostic signals.

Question

Accepted Answer

1. The common pattern is **"Hardly had ... when ..."** for two quick events.

2. **"When"** links the first event to the second correctly.

3. **"Than"** is comparative and wrong. **"Before"** or **"until"** change the time meaning.

4. So **(b) when** fits the sentence.

FINAL_ANSWER: b

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Answer

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