在人群现场通过人为控制研究因素而验证或证实假设的方法属于
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The main function of the liver is to ______.
A. clean most toxins from alcohol
B. metabolize fat from the bloodstream
C. clean most toxins from bloodstream
D. dispose of poisons from tobacco at once
根据我国法律的规定,担保方式主要有保证、抵押、质押、留置、定金等五种,其中()是人的担保。
A.butB.howC.sinceD.now
A. but
B. how
C. since
D. now