Answers

2016-02-23T16:21:04+05:30
if we dont left some space then the oil will overflow
                 to prevrent it from overflowing we left some space at the top
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2016-02-23T19:03:04+05:30
In oil tanker some space is left over the top because the oil may spill out if there is no gap left. To get rid of the spilling of the oil the space is left at the top.
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