B) -1
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解析：
令x=2,有
f(2) + 2f(1/2) = 3*2 = 6 .(1)
再令x=1/2,有
f(1/2) + 2f(2) = 3*1/2 = 3/2 .(2)
2*(2) - (1) 得,
3f(2) = -3
从而,f(2) = -1