[单选题]
A 15.0-mW laser puts out a narrdnj5 n.5 up;kyow beam 2.00 mm in diameter. What is th1bo)f cri c) tfljb(/s +8cnvl(( 3male average (rms) value of th 8n(f)m1 v cs3+ rc /oilb()tlajcbf(le magnetic field?
A. $4.5 \; mT$
B. $6.4 \; mT$
C. $9.0 \; mT$
D. none of the given answers
