He has been working on and off for several years on this research project. A. at intervals B. continuously C. rarely D. painstakingly

The correct answer is A. at intervals.

The phrase “on and off” means “occasionally or intermittently.” This is consistent with the fact that the man has been working on the research project for several years, but has not been working on it continuously.

The other options are incorrect because they do not accurately describe the man’s work habits.

• Option B, “continuously,” is incorrect because the man has not been working on the research
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