题目内容

He was observed ________ into the house and helped himself to find what he liked before the owner came back.


  1. A.
    to slip
  2. B.
    slip
  3. C.
    slipping
  4. D.
    slipped
A
本题考查observe的用法.observe sb.do sth.用于被动形式时,要加不定式符号to,即sb.be observed to do sth.B项前没有to; C、D项也都不符合这一用法.
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