Which of the following is a major characteristic of meiosis I（）
A. Splitting of the centromere
B. Pairing of homologous chromosomes
C. Reducing the amount of DNA to IN
D. Achieving the diploid number of chromosomes
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The purpose of contrast examination is to（）
A. increase the artificial contrast of organs and tissues
B. increase the density of organs and tissues
C. reduce the density of organs and tissues
D. increase the natural contrast of organs and tissues
E. change the structure of organs and tissues
The penetrating power of X-rays in different parts of the body, arranged from strong to weak, are as follows（）
A. gas, fat, liquid and soft tissue, bone
B. gas, liquid and soft tissue, fat, bone
C. bone, fat, fluid and soft tissue, gas
D. fat, gas, liquid and soft tissue, bone
Which of the following methods of introducing contrast agents is incorrect（）
A. Inhalation
B. Oral
C. Perfusion
D. Puncture
E. Intravenous injection
The common mediastinal tumor in the hilum is___（）
A. lymphoma
B. teratoma
C. retrosternal goiter
D. thymoma
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