从每题所给的A、B、C、D四个选项中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
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
24. You might wonder how it________to live without eating for so many days.
A．keeps B．Succeeds C．manages D．tries
