A .f1(t)*f2(t)←→F1(jω)F2(jω)
B .f1(t)+f2(t)←→F1(jω)F2(jω)
C .f1(t)f2(t)←→F1(jω)F2(jω)
D .f1(t)/f2(t)←→F1(jω)/F2(jω)