There is a thin biconvex lens having a focal length of 20 cm. If an object is moved to 50 cm from a distance of 25 cm in front of it, then the magnification of its image changes from ( m₂₅ ) to ( m₅₀ ). Find ( m₂₅:m₅₀ ).
a) 1:2
b) 1:3
c) 1:4
d) 1:5