Two long parallel straight conductors carry currents i1 and i2(i1> i2) . When the current are in the same direction, the magnetic field at a point midway between the wires is 20 mu T . If the direction of i2 is reversed, the field becomes 50 mu T . The ratio of the currents i1/i2 is 7/K. Then, the value of K is then ...........
A. 4
B. 3
C. 5
D. 6