设正方形的边长为xmm,
则AK=AD-x=80-x,
∵EFGH是正方形,
∴EH∥FG,
∴△AEH∽△ABC,
∴
EH
BC=
AK
AD,
即
x
120=
80−x
80,
解得x=48mm,
故选C.