—Where did you get this book?—I picked it up from a second-hand bookshop if you ______ know.
He is only thirty, but has got _____.
When _______a hand, one should say “Thank you” or “It’s very kind of you.”
Some students think it is ___ to spend some time surfing the Internet, but we shouldn’t be too dependent on it.
Not until his mother came back_______.
—What made Tom so sad? —_______.