a) To change 125 mm into centimeters:
$125 \, \text{mm} = 12.5 \, \text{cm}$

b) To change 84 cm into millimeters:
$84 \, \text{cm} = 840 \, \text{mm}$

c) To change 890 cm into meters:
$890 \, \text{cm} = 8.9 \, \text{m}$

d) To change 4.8 m into centimeters:
$4.8 \, \text{m} = 480 \, \text{cm}$