This is a Sentence Equivalence question, where you need to choose two words that are synonyms and logically complete the sentence.

Breakdown of the Blank

The sentence describes the nature of the "skirmish" (a minor fight) between the armies, based on the actions they take afterward.

The key clue is the second half of the sentence: "with each side taking much needed rest during the respites."
- Skirmish: A fight.
- Respite: A short period of rest or relief from something difficult or unpleasant.

If the armies are taking rests during respites, it means the fighting is not continuous. The skirmish must have occurred in bursts or stops and starts, allowing for those periods of rest.

Therefore, the blank must be filled with two synonyms meaning intermittently or with breaks.
- A. retroactively: Relating to or dependent on a past event. This describes time direction, not frequency.
- B. spasmodically: Occurring in brief, irregular bursts; intermittently. This is a strong fit.
- C. invasively: Tending to intrude; aggressively. This describes the manner of fighting, not the frequency.
- D. incessantly: Without interruption; constantly. This is the opposite of the required meaning, as it would preclude "respites."
- E. spuriously: Not being what it purports to be; false or fake. This describes the quality of the skirmish, not its temporal pattern.
- F. irregularly: Not occurring at regular intervals; intermittently. This is a perfect synonym for "spasmodically" and fits the idea that the fighting paused to allow for rest.

The two words that mean occurring in bursts or intermittently are spasmodically and irregularly.

The correct choices are B. spasmodically and F. irregularly.