He was observed ________ into the house and helped himself to find what he liked before the owner came back．A.to slipB.slipC.slippingD.slipped
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A解析本题考查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．