[单选]
How can he do so much work? He ________ stay late at the office every evening and take work home at weekends．

It is a small country, yet has a lot of________ resources． potential。 unknown。 secret。 unlimited。 What we have to take________the courses offered only for this term． are。 is。 have been。 has been。 John________  he could improve his exam results, but he did not have enough time to study． knows how。 knows that。 knew how。 knew that。 After________you have told me I think you should see a doctor that。 what。 which。 if。
How can he do so much work? He ________ stay late at the office every evening and take work home at weekends．