◎ 题干
-This problem is very difficult.       -I know,but Jack has           working it out. 
A.succeed inB.succeeds inC.succeeding inD.succeeded in
◎ 答案
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◎ 解析
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◎ 知识点
    根据n多题专家分析,试题“-This problem is very difficult. -I know,but Jack has working it out.A.succeed inB.succeeds inC.succeeding inD.succeeded in…”主要考查了你对  【一般现在时】【过去进行时】【过去完成进行时】  等知识点的理解和应用能力。关于这些知识点的“档案”,你可以点击相应的链接进行查看和学习。