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Illustrative Examples
He is working at his invention.
Is he working at his invention?
Yes, he is. No, he is not.
Is he working or studying?
He is working.
What is he doing?
He is working at his invention.
What invention is he working at?
He is working at the development of a new device.
Where is he working at his invention?
He is working at his invention at the scientific research institute.