[单选题]
Charge +2q is placed ats.;nq 8 h;3(gb y.c dpojr)ber bifc-( the origin and charge -q is placed at x = 2a. Where k xsfrku/e d/z6-2z r4can a third positive charges//kfru64x2- kde rzz +q be placed so that the force on it is zero?
A. $3.4 \; a$
B. $6.8 \; a$
C. $8.6 \; a$
D. $9.3 \; a$
