Same current i is flowing in the three infinitely long wires along positive x-,y- and z-directions. The magnetic filed at a point (0,0,-a) would be
A. `(mu_(0)i)/(2pia)(hatj-hati)`
B. `(mu_(0)i)/(2pia)(hati+hatj)`
C. `(mu_(0)i)/(2pia)(hati-hatj)`
D. `(mu_(0)i)/(2pia)(hati-hatj+hatk)`