选择合适的样品预处理方法时应注意()。
A.不改变待测物的状态
B.提高测量灵敏度
C.与选择的分析测量方法相适应
D.完全消除干扰物质
参考答案:A, B
_______ of the land in our city ____ covered with trees and grass.
A. Two fifths, is B. Two fifth, are C. Two fifths, are
声波自空气到内淋巴所衰减的能量约为:()
A.10dB
B.20dB
C.30dB
D.40dB
E.50dB