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The answer is artificial reef. Its a man made underwater structure typically built to promote marine life areas with a generally features bottom to control erosion, block ship passage or improve surfing. Artificial reef improve marine life in a certain area and deprive a new location for fishermen to fish and which works to ease the pressure on natural reefs.
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The answer to the missing blank above would be 'the Mongols'. The Mongols invaded Russia around 1300s, which arose to economic downfall and overall negative effect to Russia. Because of this, the Russian princes convened and fought to drive Mongols out of the country.
The Han Chinese are the ethnic majority in China<span />
