A. contradictory theories keep emerging all the time
B. new information is constantly coming in
C. the direction of their development is difficult to predict
D. our knowledge of the physical world is inaccurate
5. We know less about the astronomical universe than we don about any social system because ______.
A. theories of its origin and history are varied
B. our knowledge of it is highly insecure
C. only a very small sample of it has been observed
D. few scientists are involved in the study of astronomy
答案:ACDAD
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