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【单选题】
设连续型随机变量X的概率密度函数为 则常数b=( )A.
e+1
B.
e
C.
e-1
D.
e-0.5
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举一反三
【单选题】Which of the following statements best supports the claim that organisms share fundamental processes as a result of evolution?
A.
All organisms that are introduced into new environments have the capacity to fill vacant ecological roles.
B.
All organisms have the ability to utilize oxygen to harness energy from the chemical breakdown of organic compounds.
C.
All organisms share a genetic code organized into triplet codons, making it possible for one organism to express a gene from another organism.
D.
All organisms possess structures such as chloroplasts and mitochondria within their cells that reflect past symbiotic relationships between prokaryotic precursors.
【单选题】In the brain, only __________ of the nervous system that provide nutrients, support, and insulation for neurons—express glutamine synthetase.
A.
neurons
B.
astrocytes
C.
stem cells
D.
glial cells
【多选题】Commonly used reporter genes that induce visually identifiable characteristics usually involve fluorescent and luminescent proteins, including:
A.
the gene that encodes jellyfish green fluorescent protein (GFP), which causes cells that express it to glow green under blue light
B.
the enzyme luciferase, which catalyzes a reaction with luciferin to produce light
C.
the red fluorescent protein from the gene dsRed
D.
the
E.
coli lacZ gene, which encodes the protein β -galactosidase. This enzyme causes bacteria expressing the gene to appear blue when grown on a medium that contains the substrate analog X-gal
【多选题】下面关于unix的描述正确的是（）
A.
cat用于显示文本文件内容
B.
more用于分页显示程序或者文本
C.
使用chmod777可以改变文件访问权限为属主、用户组、其他人都可读可写可执行
D.
使用chmod ugo+rwx abc.c表示改变文件abc.c访问权限为属主、用户组、其他人都可读可写可执行
【单选题】某文件的权限是 -rwxr--r-- ,则下面描述正确的是
A.
文件的权限值是 755
B.
文件的所有者对文件只有读权限
C.
其他用户对文件只有读权限
D.
同组用户对文件只有写权限
【多选题】大气中NO,NO2和O3之间存在着的化学循环是大气光化学反应的基础,下面那些化学反应与之相关。
A.
NO2 + hv→NO + O●
B.
O●+ O2 + M→O3 +M
C.
O3 + NO→NO2 + O2
D.
以上均不是
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