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【单选题】
嘌呤环中的氮原子来自( )A.
乙酰天冬氨酸
B.
谷氨酸
C.
丙氨酸
D.
cGMP
E.
谷氨酰胺
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举一反三
【单选题】The passage suggests that the fact the 'only one community cancer cluster had ever been traced to an environmental cause' (Para. 3)is most likely due to the
A.
methodological difficulties in analyzing community cancer clusters
B.
reluctance of epidemiologists to investigate environmental factors in cancer
C.
lack of credibility of citizen activists in claiming to have identified cancer agents
D.
effectiveness of regulations restricting the use of carcinogens in residential areas
【多选题】下列哪几项内容属于自然经济的发展阶段( )
A.
上古时期，人们获取现成食物，不懂“生产和再生产”。
B.
母系氏族公社时期是原始农业的开端，出现自给自足的自然经济。
C.
夏末至商代，社会经济单位由原始氏族公社逐渐演变为奴隶主贵族庄园。
D.
周代初期，社会经济单位为奴隶主贵族庄园或封建领主庄园。后期，井田制瓦解。封建领主变为封建地主，农奴转化为佃农。以“男耕女织”为特征的家庭农业和小手工业生产为经济单位的自然经济形成并发展。
【单选题】_________ the fact that many drivers are ever trained, we cannot stress too much theimportance of traffic safety.
A.
Thanks to
B.
Despite
C.
As
D.
Although
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