以15N、14C标记氨基酸饲养犬，然后观察其尿排泄物中含15N、14C双_爱下问搜题

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问题单项选择题

以¹⁵N、¹⁴C标记氨基酸饲养犬，然后观察其尿排泄物中含¹⁵N、¹⁴C双标记的化合物。可发现含量最多的是

A．尿酸
B．尿素
C．肌酐
D．β-丙氨酸
E．尿胆素

答案

参考答案：B

解析：正常情况下，氨基酸的分解代谢产物氨主要在肝脏转变为尿素并随尿液排除体外。尿素分子中的C、N均来自氨基酸，因此在尿排泄物中含¹⁵N、¹⁴C双标记的化合物主要是尿素。

多项选择题

政府采购应当遵循的基本原则是（）。

A.公开透明原则

B.公平竞争原则

C.公正原则

D.诚实信用原则

完形填空

While attending a conference, I returned to my hotel room late one evening. The overhead light outside my door was burned out and I had       finding the keyhole. When I       to open the door, I       around the wall for a light switch. I found a plate where a switch was       installed... but no switch!
Not discouraged easily, I remembered       a lamp by the bed when I deposited my luggage      in the day. I found the bed in the       and then the lamp, but when I switched it on, nothing     ! I thought that perhaps if I opened the curtains I might be able to use whatever light comes in from the       to find another lamp. So I  my way slowly across the room to the curtains and... no drawstring（拉绳）! I finally stumbled（跌跌撞撞地走）around        I found a desk lamp which actually     !
That evening I discovered in a whole new way just how dark the world       be and how   necessary light is! But even more necessary than       light is the light that shines from people --the light of       and faith. Because, for many people, the world is a dark and       place. For someone today just may be stumbling in discouragement or sadness or fear and in        of some light. So let your light shine. Whatever light you       may be a beacon（灯塔） of hope and encouragement. And if you feel that your light is       a candle in a forest remember -- there isn’t enough darkness in all the world to       the light of one small candle.

B．psychological

A．not more than

C．no more than

D．rather than