4．Many bad habits can be especially difficult to cure because they are likelyto have been formed at a very young age．（form）．
許多壞習慣特殊氣味難以糾正，因為它們可能是人們在年幼時就形成了的．

分析 be likely to屬于固定短語，由于habits和form之間是被動關(guān)系，又因為是在"年幼時就形成了的"，綜合這三點，空里的內(nèi)容應(yīng)該用不定式的完成時態(tài)，即"to have been done"結(jié)構(gòu)．

解答 答案：to have been formed

點評 考查現(xiàn)在完成時的被動語態(tài)．根據(jù)時間狀語確定時態(tài)，由于固定結(jié)構(gòu)"be likely to"，所以用不定式的完成時態(tài)．

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