f(x+4)=-f(x+2)=-(-f(x))=f(x)此为函数周期性,周期为4
f(7.5)=f(3.5)=f(-0.5)(周期性)
再由奇函数得f(-0.5)=-f(0.5)
f(7.5)=-f(0.5)=-0.5
选B