It’s not easy for the expert to pig flu.in humans from regular human seasonal influenza.

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(答案→)A. distinguish
B. separate
C. identify
D. recognize
解析：A考查动词词义辨析。句意：对专家来说不容易把人患的猪流感和普通流感区分开来。Distinguish---from把----与---分别开来；separate----from---把---与----分隔；identify辨别；recognize认出。