[单选题]
How far from a lens of focal eq4 w a(du6xe 2f5x:na+iewu ,dcnx4 vq6o *tk7p t8 b;u ,+p :tsjfz*ocv5ao8length 50 mm must the object be placed if it is to form a virtua*p ttqb,+af7*cpvzokoc: j xnst 5ou d4 ;688vl image magnified in size by a factor of three?
A. $33 \; mm$
B. $42 \; mm$
C. $48 \; mm$
D. $54 \; mm$
