17.This disease is second only _______ heart attack as a cause of death all over the

17.This disease is second only _______ heart attack as a cause of death all over the
world.
A.to B.of C.with D.from
18.The word is new to us.You'd better ______ in the dictionary.
A.look at it B.look for it C.look it up D.look it out
19.If there is no one _______ him with the preparation,he will have to put off the
meeting.
A.help B.to help C.helps D.helped
20.As soon as she _______ open the letter,Mrs.White will find out what has happened to
her husband.
A.tears B.is tearing C.will tear D.has torn
21.______ the government argees to give extra money,the theatre will have to be closed
next month.
A.Unless B.If C.Since D As
答案是17.A18.C
19.B20.A21.,可我不理解,麻烦帮我解答下