The line `lx+my+n=0` intersects the curve `ax^2 + 2hxy + by^2 = 1` at the point P and Q. The circle on PQ as diameter passes through the origin. Then `n^2(a+ b)` equals (A) `l^2+m^2` (B) `2lm` (C) `l^2-m^2` (D) `4lm`