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BEIJING, Oct. 13 (Xinhuanet) -- A recent sudden temperature drop in most areas of China has set off fears of a possible return of severe acute respiratory syndrome (SARS) and the country has mobilized to prepare for another outbreak.   North China's Tianjin Port resumed a temperature reporting system on Sunday. Any passengers through the port with a temperature above 38 degrees Celsius would be provided medical observation and reported to relevant authorities.  Zhong Nanshan, a well-known anti-SARS scientist during the last outbreak, said it was unlikely the SARS virus would die out natural对于牛来说，下列因素中属于非生物因素的是 [　　] A．牛的食物牧草B．同时吃草的羊 C．牛群中的其他羊D．新鲜的空气