[单选题]
Charge +2q is placed at the ad73-m7 leq 0zj).v0pdl a4-wau ymvz originkr :e;ou(3 r1w6o )c.ylrwbqw and charge -q is placed at x = 2a. Where can a third positive charge +q be placed so that the forc(l3.owy16wo ur:c kbe ;rq)w re on it is zero?
A. $3.4 \; a$
B. $6.8 \; a$
C. $8.6 \; a$
D. $9.3 \; a$