[单选题]
Two very thin lenses, eaj .;.8,sr* *jfdrp s-ajwreaw chaum.cffb .+js903bfs with focal length 20 cm, are placed in contact. What is the jmscf0 9 bb+3.suffa.focal length of this compound lens?
A. $40 \; cm$
B. $20 \; cm$
C. $15 \; cm$
D. $10 \; cm$