1．The world we live in is full of ____________(无法解释的)mysteries.
答案：unexplained
2．The economy of that area is developing at an ____________(令人惊叹的) speed.
答案：amazing
3．The ____________(消失) of the boy made the whole family unhappy.
答案：disappearance
4．The problem seemed very ____________(令人困惑的)．Few students could work it out.
答案：puzzling
5．As an old teacher he has ____________(见证) the great changes that have taken place in the school in the past thirty years.
答案：witnessed
