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【单选题】
如下图所示，DDS的基本结构组成：相位累加器、相位调制器、波形数据存储器和DAC电路构成。【图片】关于相位累加器，以下说法错误的是（ ）。A.
相位累加器是DDS的核心，也称地址计数器，由地址加法器和寄存器组成。
B.
地址加法器的有两个输入端，一个是频率控制字输入端，另一个与地址寄存器的输出端相连。
C.
如果基准时钟频率是100MHz，需要最低1kHz的精度，相位累加器必须产生10MHz/1000Hz=100000个地址，则m应等于16。
D.
如果把相位累加器去掉，就没办法改变输出波形的频率了。
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【多选题】What are the functions of reverse transcriptases?
A.
Reverse transcriptases catalyze three different reactions: (1) RNAdependent DNA synthesis, (2) RNA degradation, and (3) DNA-dependent DNA synthesis. Like many DNA and RNA polymerases, reverse transcriptases contain Zn 2+ .
B.
Each transcriptase is most active with the RNA of its own virus, but each can be used experimentally to make DNA complementary to a variety of RNAs. The DNA and RNA synthesis and RNA degradation activities use separate active sites on the protein. For DNA synthesis to begin, the reverse transcriptase requires a primer, a cellular tRNA obtained during an earlier infection and carried in the viral particle.
C.
This tRNA is base-paired at its 3′ end with a complementary sequence in the viral RN The new DNA strand is synthesized in the 5′ → 3′ direction, as in all RNA and DNA polymerase reactions. Reverse transcriptases, like RNA polymerases, do not have 3′ → 5′ proofreading exonucleases.
D.
They generally have error rates of about 1 per 20,000 nucleotides added. An error rate this high is extremely unusual in DNA replication and seems to be a characteristic of most enzymes that replicate the genomes of RNA viruses. A consequence is a higher mutation rate and faster rate of viral evolution, which is a factor in the frequent appearance of new strains of disease-causing retroviruses.
【单选题】根据《建设工程工程量清单计价规范》(GB 50500—2008)的规定，下列选项中，不属于按设计图示尺寸以面积计算墙面镶贴块料的是( )。
A.
石材墙面
B.
碎拼石材墙面
C.
装饰板墙面
D.
块料墙面
【单选题】What is the difference between a reverse auction and mass customization?
A.
Reverse auction is most often found in B2B whereas mass customization is often found in B2C.
B.
Reverse auction requires a bid from consumers whereas mass customization requires a bid from suppliers.
C.
Mass customization require a click-and-mortar environment; reverse auctions do not.
D.
Mass customization can sell digital products; reverse auctions sell commoditylike products.
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