21. —It’s a long time________I saw you last.
—Yes，and what a pity it is now that it will be a long time________we see each other again.
A．before；since B．since；when C．since；before D．when；before
22. By the time you have finished this book, your meal ______ cold.
A. gets B. has got C. will get D.is getting
23. One learns a language by making mistakes and ______ them.
A. corrects B. correct C.to correct D. correcting
