12.下列何者是f(t)=cos(10πt)博立葉轉換F(f)?(A)0.5[δ(f+5)+δ(f-5)](B)0.5[δ(f + 5) -δ(f-5)](C)0.5[δ(f+5) + j•δ(f-5)](D)0.5[δ(f+5) - j•δ(f-5)]