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The Silk Road began in north-central China in Xi'an (in modern Shaanxi province). A caravan track stretched west along the Great Wall of China, across the Pamirs, through Afghanistan, and into the Levant and Anatolia.

Answer (d) the greenhouse effect is primarily caused by car and trucks;

When a lake, river or other aquatic  system  becomes  eutrophic, it can have serious negative effects on other organisms like fish, birds and even people.if an algal bloom is so bad that it causes wide-spread death  in  the  water, the organisms that die will all sink to the bottom and start to decompose.