单项选择题

The underlined word "it" refers to which of the following

A．Problem.
B．Material.
C．Difficulty.
D．System.
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单项选择题
According to the passage, how did knowledge about chemical processes increase before the seventeenth century
A．Philosophers devised theories about chemical properties.
B．A special symbolic language was developed.
C．Experience led workers to revise their techniques.
D．Experts shared their discoveries with the public.
单项选择题
Which of the following statements best explains why the second of these was the more serious impediment
A．Chemical knowledge was limited to a small number of people.
B．The symbolic language used was very imprecise.
C．Very few new discoveries were made by alchemists.
D．The records of the chemical processes were not based on experiments.
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