1．—Hi, waiter.How about my dish? I ____________(wait) for 20 minutes.
—Oh, sorry madam.Just a minute, please.
答案：have been waiting
2．—Why haven’t you asked her to come here?
—She ___________(do) an important experiment when I found her and she ___________ (not finish) it.
答案：was doing；hasn’t finished
3．Leonardo da Vinci(1452—1519) is said ____________(buy) birds kept in cages in order to have the pleasure of setting them free.
答案：to have bought
