Two parallel long straight conductors are placed at right angle to the meter scale at the $2 \mathrm{~cm}$ and $6 \mathrm{~cm}$ marks as shown in the figure. If they carry currents $i$ and $3 i$ respectively in the same direction, then they will produce zero magnetic field at
(a) zero mark
(b) $9 \mathrm{~cm}$ mark
(c) $3 \mathrm{~cm}$ mark
(d) $7 \mathrm{~cm}$ mark