During the period 1490—1979 the main progress mentioned in this passage was______. Soebe’s invention was net a perfect one, bec

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问题 During the period 1490—1979 the main progress mentioned in this passage was______.
Soebe’s invention was net a perfect one, because______.

选项 A、too great a supply of air could result in a sudden rise to the surface.
B、a sudden dive into deep water could cause the crushing of the body.
C、the air pipe attached to a machine could be above the surface.
D、both A and B.

答案D

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