[单选题]
A 15.0-mW laser puts out a narrow beam 2.00 mm in da *+j9x,pjl ceiameter. What is thjc:kyn 6t2j 6f43twxr e average (rms) value of 3fj6wk c2 xn :tjtr64ythe magnetic field?
A. $4.5 \; mT$
B. $6.4 \; mT$
C. $9.0 \; mT$
D. none of the given answers
