A　[细节理解题。根据第一段"When European settlers arrived in...dying from smallpox(天花)infection."可知欧洲殖民者进入北美洲时，也带去了传染病，所以A项正确。]

　　2．Genes help determine many things of a person EXCEPT ．

　　A．a person's appearance

　　B．a person's skin color

　　C．a person's height

　　D．a person's success

　　D　[细节理解题。根据第三段"Genes help determine a person's...chance of getting a disease."可知基因可决定一个人的外表、肤色、身高，或得某种疾病的可能，而不是一个人的成功。所以D项正确。]

　　3．The scientists got the result of the study by ．

　　A．following genes' change over time

　　B．studying genes of ancient people

　　C．analyzing large amounts of data

　　D．finding out genes in the cells

　　C　[细节理解题。根据第五段"Using statistics，which includes...Europeans' arrival."可知通过分析对比数据，得出欧洲人到美洲前后美洲居民的数量，从而得出最后的结论。所以C项正确。]

　　4．What can be the BEST title of this passage?

　　A．Scientists Working on Genes

　　B．Genes Tell an Old Story

　　C．Europeans' Arrival in America

　　D．DNA Changes Over Time

　　B　[标题归纳题。本文主要讲的是研究基因，通过数据分析的方式来找到历史事件的证据。A项意思是"研究基因的科学家"；C项意思是"欧洲人来到美洲"；D项意思是"DNA随时间变化"，表述都比较偏颇，与本文主要内容不符。所以B项正确。]

IV.完形填空

A few days ago I saw two baby ducklings that were lost in the grass. 1 ，one of