A.48.0mmB.52.0mmC.40.0mmD.20.0mm
A.1/f=1/f1+1/f2+1/dB.1/f=1/f1+1/f2-1/dC.f=f1f2/(d-f1-f2)D.f=f1f2/(f1+f2-d)
A.-f1/f2B.-f2/f1C.-f2/(f1+f2)D.-f1/(f1+f2)