During the period 1490~1979 the main progress mentioned in this passage was ______. Siebe’s invention was not a perfect one, be

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问题 During the period 1490~1979 the main progress mentioned in this passage was ______.
Siebe’s invention was not 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

解析 由第三段中的But there were two dangers to...One was...The other was...可知,hard-hat suit有两大危险,答案选项表述的正是这两个方面,故正确。
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