[单选题]
A 15.0-mW laser puts out 0t foo/u:ly;dsom 6:sa narrow beam 2pjxm66a(kp - xoeq ; n oqw6h)-4acb)g.00 mm in diameter. What is the average (rms) value of the oxk(6q p4) gpcwxe;h6 6o-ab )a-jqmnmagnetic field?
A. $4.5 \; mT$
B. $6.4 \; mT$
C. $9.0 \; mT$
D. none of the given answers
