We need to learn __________ be polite when we make requests.a. what to b. why to c. how to d. whether to
Paul doesn’t know _________ about his problems.
--Who _______ the Harry Potter series________ ? -- J.K.Rowling.
We know that about seventy percent _____ the earth _____ covered by water.
The question is _________ .
-- Must I return the book this week? -- No, you ______. You can _____ it for 20 days.