①What was once regarded as impossible has now become a reality．

　(what为"所......的事"，相当于"the thing that...；all that...；everything that...")

②After ________ seemed a very long time，I opened my eye and found myself in bed．（M ET'93）

　A.what B.when C.that D.which

　(what相当于"the time that"，表示"......时间")

③He is not what he was a few years ago. Who is it that has made Fred what he is now？

　(what表示"......的人"，相当于"the person that...")

④What is now the North Sahara Desert was once a civilized world．

　(what表示"......的地方"，相当于"the place that...")

⑤Our income is now double what it was ten years ago

　(what表示"......的数目"，相当于"the amount ／number that...")