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【单选题】
核酸具有紫外吸收能力的原因是A.
嘌呤和嘧啶环中有共轭双键
B.
嘌呤和嘧啶环中有氮原子
C.
嘌呤和嘧啶环中有硫原子
D.
嘌呤和嘧啶连接了核糖
E.
嘌呤和嘧啶连接了磷酸基团
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举一反三
【单选题】Which of these statements concerning positive feedback is correct?
A.
Positive-feedback responses maintain homeostasis.
B.
Positive-feedback responses occur continuously in healthy individuals.
C.
Birth is an example of a normally occurring positive-feedback mechanism.
D.
When the cardiac muscle receives an inadequate supply of blood, positive-feedback mechanisms increase blood flow to the heart.
E.
Medical therapy seeks to overcome illness by aiding positive-feedback mechanisms.
【简答题】'Contact with non-Roma is regarded as potentially polluting.' probably means ______.
【单选题】The following statements concerning the diaphragm are correct except
A.
The right crus provide a muscular sling around the esophagus and possibly prevent regurgitation of stomach contents into the esophagus.
B.
On contraction, the diaphragm raises the intra-abdominal pressure and assists in the return of the venous blood to the right atrium of the heart.
C.
The level of the diaphragm is higher in the recumbent position than in the standing position,
D.
On contraction, the central tendon descends, reducing the intrathoracic pressure.
E.
The esophagus passes through the diaphragm at the level of the eighth thoracic vertebra.
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