Study the following and pick up the incorrect statements:
I. In commensalism both partners are benefitted due to each other.
II. In amensalism no partner is benefitted.
III. In parasitism, one partner is benefitted and the other is harmed.
IV. In competition both the partners are benefitted.
Two concentric thin circular rings of radii 50 cm and 40 cm each, carry a current of 3.5 A in opposite directions. If the two rings are coplanar, the net magnetic field due to the two rings at their centre is: