[单选题]
How far from a lens of focal length 50 mm must the objecl4gk kvb;wl4r4 rp r3(t be placq6 y9oskoo43x ed if it is to form a virtual image magnified in size by 3o q4oxo k9ys6a factor of three?
A. $33 \; mm$
B. $42 \; mm$
C. $48 \; mm$
D. $54 \; mm$
